Oh golly, yes. 12 GA will be just fine, even if it is a really long extension. Why do I say that? Because the Romex serving your outlet is almost certainly 12 GA, and possibly the smaller 14 GA. What's more the length of Romex back to your breaker box is probably much longer in length than your extension cord.
For most situations, people get in trouble when using extension cords designed for lamps. And even then, you will probably only have a problem with large inductive loads, like fractional horsepower motors. You find these motors on air compressors, big saws, etc. When they start, they are, for a brief moment, essentially a short circuit, and require a huge amount of current. If they don't get that current, they will stall. I have a very old, piston-type air compressor, and it will not start if I use a long extension cord.
Heating elements, like those in an electric smoker, could care less about your extension cord.
As an example, using this calculator, if you have a 1,200 watt heating element, it will draw 10 amps. From the calculator, if you use a 20-foot 12 GA extension cord, the drop will be a little over 0.5 volt. That is absolutely nothing. I have a UPS on this computer that has a front-panel display of line voltage coming into the computer, and it goes up and down by 1-2 volts all the time. Thus, the variations in the voltage from the power company far exceed the voltage drop you are going to introduce with an extension cord.
The other issue is heat, because under extreme circumstances, using a really small gauge extension cord, the cord might get really hot and cause a fire. However, even with a 16- or 18-gauge extension cord, this is not likely to be an issue. The real fire hazards come from damaged cords where only a few strands of copper remain, usually at the point where the plug meets the cord (they get damaged when you pull the plug out from the cord, rather than grasping the plug itself).