Gil has two attacks: Gob and Ea (plus Enkidu, but that's a bonus). Now, you can't pretend he goes "anti-world" against opponents like Shirou or generic ones (like lancelot of which he doesn't know anything about his abilities) right from the start.
Nobody, not even the good ones, does that. Next step, I'll say "Arturia is a moron because she doesn't excaliblast everyone instead of going crappy C-rank Invsisible Air". And if GoB (+ Enkidu) is enough to kill Heracles alone, why should he need to bring out Ea? Yes, GoB is his default attack, but it's perfectly serious and lethal.
Not being serious would be like during the fight at the bridge, where he just tanks Saber's sword with the armor until "derp, if this keeps going on she'll kill me".