To keep your GL on Titan EM:

First jump: you must have recently completed an attack within a second of his jump, and run towards the center as he's falling. You can lose GL if he jumps before you get your attack off, it's semi-RNG factored since crits and such can push him beyond your ability to predict his jump to the second.
Second jump (heart): can't normally keep it, but if your team pushes during Gaols, you will keep it though your team will be in a much worse spot.
Third jump (post-heart): only way to keep is if you push him during circle bombs and attack the center bomb.
Fourth jump (post-super bombs): attack bombs to keep up your rotation, just make sure you don't get hit by an explosion.

Technically, if you don't get Gaoled, you can indeed keep up GL3 through the whole fight. It requires certain DPS (not necessarily good DPS, semi-slow in fact) to do. That being said, in no way is it guaranteed.