In the first movies, Gamera pulling his Big Damn Heroes moment at the bridge where he reveals he can shoot fire balls. And then when Asagi gathers power, with her dad's aid, to revive Gamera and then just a little bit more for him to fire one last fireball and finish off Super Gyaos.
In the second, Gamera arrives at Ashikaga and joins the battle with the SDF against the Legion. Gamera flies in, lands, and while sliding backwards launches a barrage of fire attacks against Legion.
And the bit where he rips off Legion's horn with just his claws.
Gamera destroying Legion with his Mana Beam.
In the third, Gamera ripping Ayana out of Iris' chest then blowing his own arm off to catch Iris' fire balls and form them into a flaming fist to obliterate Iris. Also, the final scenes.
The reveal of the new trailer which came out of no where after everyone was sure the project was cancelled.