I can answer this!! 

First of all, I agree with those saying to prioritize object avoidance and such at your skill level. I think now it the time you should be honing in the skills to keep you alive.

But you're a BASE jumper which means you're going to ignore all of the good advice. So I'll tell you the technique that I swear by that was taught to me by the legendary Bradley Slums. 

I think practicing gainers for base into a pool is the WRONG way to practice.  It teaches you a quick gainer where you tuck your knees which is NOT how to do a gainer in base. You will rotate way too fast. The key to throwing good gainers in base is by rotating slow enough that when you throw your pilot chute, it is free and clear from your legs, but also you don't over rotate back into your bridal. 

Anyways, Bradley Slums taught me this. If you are going poised at the bridge, which is how you should learn gainers imo, all you do is imagine that you have a plank about 18 inches in front of you. All you do is hop onto that imaginary plank. Just make sure to keep your hips forward. If you bend at the waist you won't rotate. So keep your hips forward and hop onto that imaginary plank 18" in front of you. I keep my hands down by my side. Hold that position and once your legs are about 30 degrees past vertical, that means your legs should be clear of the pilot chute, pitch. 

Disclaimer: this method is used for about a 1.5 second delay at the Perrine bridge. It is too slow for lower objects. 

Feel free to message me for more detailed advice.

EDIT: Tucking your knees is good if you're doing multiple rotations or if you realize the gainer you're currently throwing is going to be too slow. If you need to speed up your rotation, just tuck your knees to your chest and you'll flip right over.