Rory totally neutralized Woodley. Woodley looked terrified and paralyzed for the entire fight. GSP probably believes he can do the same thing with his jabbing and karate.
Only reason I can think of why GSP wants to come back now at this point in time.
The fight doesn't make that much sense otherwise. Why fight for the championship if you're already 35 and will have to retire again soon anyway? Why not take a super fight against an equally old legend?
It's because a good opportunity came along: a champion that, in GSP's mind, he KNOWS he can beat. The thought of winning the title for a 3rd time from a younger, hungrier fighter would be Rocky-esque and would make him frontrunner for UFC GOAT now that Anderson and Jones both tarnished their legacies with PEDs. And you know GSP is all about his legacy.
So my theory is that GSP feels confident because of what his teammate and protege Rory Mac did to Woodley, and he thinks he can follow that blueprint with Firas's guidance to solidify his legacy.