Okay, let's clear things up. You press R1 to activate Squall's critical gunblade attack. The best place to press the button is right when you hear the gunblade slash sound byte. Watch it a few times to get a feel for the delay before you hear it, then start pressing it right as you're expecting to hear the sound. Once you do it once or twice you'll be able to do it almost 100% of the time. It's not hard to master once you realize the timing.

For Renzokuken just watch the indicator at the bottom. The lines indicate when Squall is going to strike, so just press the R1 trigger whenever there is a line passing through the strike area. Really easy stuff.