so my power button recently died and was wondering if I could somehow use gestures to assign to lock screen? (would like to make an X on the screen to lock the screen.)
With Assistive Touch you need to tap the device button, from there you can lock it. You could also jailbreak your phone and use Activator so assign a sleep button alternative.
Edit: Now I see that you said gestures, I don't think so, only with jailbreak. Sorry.