Hi! I think a good thing to remember, especially at first, is that your drawing doesn't need to look good or finished, especially as you're learning faster gestures, which are hard as hell to make look good. I usually try to think of them as a warm up, or practice through sheer exposure. You're essentially trying to improve your ability to take down the information of a body easily.
It's usually easiest to add structure by figuring out where the biggest shapes are and then working from there. People have varying ways of portraying them, but generally speaking structure starts with the rib cage, the pelvis, and the head. The rib cage gives you the angle of the shoulders, the pelvis the angle of the hips, and together they give you the movement of the torso. From the torso, you add the limbs--which are terribly fiddly and so hard to do in a gesture. I'm not sure about everyone else, but personally I give them 0 structure during 30 minute and 60 second gestures--I just don't have the time. However for 3 and 5 minute gestures, I'll indulge a bit. I find it most useful to draw (lightly) where the big joints are (the caps of the shoulders, the place where thighs meet pelvis) and then from there (still lightly) fill in the big underlying muscle shapes, which are always rounder and softer than I often want to make them unless I put ovals underneath. This is a bit hard to explain with words so here's a picture of what I mean:
https://photos.app.goo.gl/q43mVbPFQpASsnXg7
Calves are the place I put them most often because I struggle with them the most, but other good places for them are the shoulder caps, biceps, and thighs. They just give you a guide of what is holding up the "lines" you're going to draw more firmly over top. From there I find it's mostly practice and preference. When do you like to use structure and when do you find you don't need to? Do you like more or less structure? What pieces of structure do you find give the most info when you're short on time?
A great way to get more comfortable with the underlying structure is to do anatomy practice of underlying muscles. Find a rendering of a person without skin (gross sounding but... yeah) and then draw simple ovals and circles to put together a very silly looking balloon person, and then once you feel like you understand generally where the big parts of people are, start using them in your gestures before you put down your hard lines. Try doing more or less, or for certain types of poses, or for certain areas that you find are hard to capture enough info about in a quick gesture otherwise.
I hope this helps some. Good luck!