You got it. The concept is that what's in your feeder's gut is what goes into your dragon's gut. So, for about 2-3 days prior to offering the feeder insect to your dragon, only offer it good quality food items which are appropriate in a dragon's diet.
When I bred crickets, my gutload was collard greens/apples/carrots with multivitamin powder on them. You may find you have to compromise based on the bug's tastes. For instance, I would've preferred my crix to be full of mustard greens (a bit lower in oxalate content), but the crix didn't like them.
Now that I have roaches, I find this to be a bit of a problem. The core of most roach diets is dog or cat food, which is loaded with grain, in addition to fat/protein, and grain doesn't exist in a beardie's diet. When I try to gutload my roaches and clear out that grain fo rthose 3 days, they will eat carrots and fruit, but all of my roaches absolutely refuse to eat greens, which are the core of the beardie diet. My dragons all eat their veggies, so it's not that big a deal, but if I ever get a picky dragon, it appears I'll never get greens into my roaches.
Anybody else have green-hating roaches?
Frank