Choosing the right drivetrain is one of the main decisions when buying a gravel bike. The single chainring (1x) drivetrain is already a standard in MTB. So what happens in the case of gravel bikes? In this matter, as in so many others related to cycling, there is no absolute truth. It all depends… It depends on the terrain where you want to ride, it depends on your budget, it depends on your pedaling style, it depends on how you feel… The good thing is that all brands offer a wide range to equip your gravel bike with the most suitable drivetrain. Shimano and SRAM have specific gravel groupsets that offer both 1x and 2x options. This is not the case with Campagnolo, where their specific group (Ekar) only comes in 1x. The design of a chainring made for 1x drivetrains is very different from that used in double chainring (2x) drivetrains. If you look closely, their teeth are deeper and alternate between two different widths. They alternate around the circumference of the chainring to mesh much better with the narrow and wide links of the chain. It's a super effective system that prevents the chain from dropping. 
Therefore, if you have ever wondered, removing the front derailleur from a double chainring drivetrain would not work as a specific single chainring drivetrain does; simply, those chainrings are not designed for that and the chain would constantly drop. We should also mention the cassette, which in this type of drivetrain has a mechanism to prevent the chain from moving around too much. All of this contributes to making the mechanism robust. As for the double chainring, there is little to say about a mechanism that has been around for 70 years. Obviously, it is more complex, since it has a front derailleur, a specific shift lever, the corresponding cables, etc…
How the double chainring affects a gravel bike
Possibly, the biggest advantage comes from the number of gear combinations, which allows for a varied range of speeds; in theory, double. But that’s in theory, because it’s not exactly like that. There is some overlap between gears. At first glance, a double chainring drivetrain with an 11-cog cassette would result in 22 different speeds. However, after eliminating duplicates, this number drops to 14, not so far from the 11 that the same single chainring drivetrain would provide. It might seem otherwise, but that’s the reality. 
It is interesting to highlight that the gear shift is much smoother than with a single chainring transmission. This is something you appreciate when your gravel routes run over favorable terrain, both on tracks and on road sections. These are circumstances in which you will seek a constant cadence, with sensations reminiscent of a road bike. Nor should you worry too much about steep slopes or when riding on trails or more technical terrain, as long as you choose a small chainring focused on this type of effort. Let's say, for example, that you opt for a double chainring of 48-31. With a generous cassette choice, you can climb almost anything. That said, are there reasons to choose a single chainring transmission? Isn't that somewhat contradictory?
How does a single chainring affect a gravel bike?
In times when less is more and design matters just as much, the single chainring is aesthetically more attractive. This should not be underestimated, although it may not be the most critical factor. Let's move on to performance-related issues. First of all, the 1x transmission contributes to the bike's lightness, since the front derailleur and one of the chainrings, among other components, are removed. It won't be decisive—barely 200 or 250 grams less on the scale—but everything counts. Another important issue is that removing the front derailleur allows you to fit wider tires. This makes a difference even compared to 2x transmissions specifically focused on gravel. This is the case, for example, with the Shimano GRX group. Its double chainring is slightly offset outward to allow for wide wheels, but it loses out to 1x transmissions, limiting maximum width to 42 mm. Some brands have tried to tackle this issue by modifying the geometry of some gravel models with longer chainstays, allowing for oversized tires even with a double chainring. But it is a complex issue, as the bike's behavior changes. The longer the bike is at the rear, the more stable and less agile it becomes. Keep that in mind. 
Another advantage of the single chainring transmission is that it only requires one shifter, on the right side of the handlebar. With the left side free, there is space to include features from mountain bikes, such as the lever to control the dropper post, in the case of gravel bikes equipped with this device. Now let's talk about gear ratios. How does it work if you eliminate half of the front transmission? First, you will need a cassette with as wide a gear range as possible. Especially at the high end, so you don't have problems facing any wall that comes up. Surely you won’t want to drag your bike uphill due to a lack of gearing. Let’s suppose you have an 11-42 cassette combined with a 42-tooth chainring. In that case, you’ll be able to keep pedaling at speeds up to around 60 km/h. Therefore, you shouldn’t worry about that aspect. As for the jump between gears, it can indeed become somewhat uncomfortable at times; especially on the road and when riding in a group. Off the tarmac, speeds are usually lower and the terrain is rougher. That’s why the jump between gears not only ceases to be a problem, but can even become an advantage, since this way we avoid wasting time (and losing rhythm) shifting from sprocket to sprocket until we find the most comfortable combination. Another relevant issue with 1x drivetrains is that the chain suffers more. From the same position, it is forced to cross over more than usual to engage all the sprockets. Narrower and more flexible models have been designed to address this, but wear is faster than with 2x drivetrains.
Which drivetrain should you choose for gravel?
Having said all this, what is the ideal choice for your gravel bike? Well, as we pointed out at the beginning, it depends. The perfect choice is the one that best suits your needs, your conditions, and your routes. If less than 50% of your rides take place on tracks or trails, then a 2x drivetrain may be more suitable. It offers a wide range that will theoretically allow you to go fast on favorable terrain and tackle steep slopes with confidence. If, on the other hand, you prefer a more pure gravel bike with the ability to fit super-wide tires, if you love simplicity, if you want to reduce some weight and adopt a slightly more robust system, then the single chainring may be your perfect choice.