While I usually encourage new members to use the forum search feature, I’ve answered questions like these before and just feel the need to give you the definitive answers you seek:
1.) Would masturbating affect gains?
No. We have members here who have steady girlfriends or wives and have sex with them frequently. We have a ton of 20-something members and you know they’re masturbating if they don’t have a girlfriend. We also have 18 year olds and they’re masturbating even if they have a girlfriend. Most of these guys are gaining. Masturbation does not seem to have any effect on gains.
2.) Would it matter if you masterbate before or after PE ( which is the better alternative?
My personal opinion is that jerking off after is better. See the next part for why.
What happens if you masterbate right before PE and what happens if you masterbate right after PE.
If you do it before you’re more likely not to want to do your whole routine. Your body will be satisfied that you’ve gotten off and may not cooperate with the things you want to do in your routine. You may be mentally “done” and not want to play with your dick any more that day. You’d have to clean up your mess before you start your exercises.
If you do it after PE you can look at it as a reward for finishing all your exercises. You will have had the strength and stamina to do your exercises that you might not have if you pulled your pud for pleasure first. You can wipe off the stuff you jelq with (Vaseline?) and your spooge at the same time.
3.) Masterbating won't affect stretching but it will affect Jelqing?
Yes. Since jelqing requires you to be semi erect masturbating before jelqing can put you into that refractory period after ejaculation where you cannot become erect. It could delay or prevent you getting hard enough to have a good jelq session.
PS - If you’re going to do it, learn to spell it correctly: masturbate.