Everyone here is complaining about Gael being the last boss or him being too "easy" for a last boss, yet everyone fails to see the true significance of this fight.
Gael wasn't an ancient godly lord or king like Gwyn and the Pygmy lords, he wasn't a respected and sung knight of legend like Ornstein and Artorias, he wasn't a gifted scholar like Vendrick's brother or Seath the scaleless. Hell, he wasn't even a free man.
He was just another slave used for wars, just a random hollow that managed to make it all the way to the end of the world out of sheer perseverance and luck. Gael is literally the same as us, the players. Someone that's decided to destroy the fake world of the gods and give the world of man a chance to truly begin.
What makes this fight truly so meaningful is that we aren't fighting a full-scale war against a "Super ultra mega dragon overlord god of the abyssmal hell" with volcanoes exploding all around the planet and fire raining from the skies as armies kill the **** out of each other (Okay, maybe a bit too exaggerated, but you get the idea) or any of that over the top action movie bull****, we are fighting someone who is like us. It's a battle between two hollows, which are probably two of the very few remaining living beings out there, that managed to outlive and defeat everything and everyone else and reach the end of the god's fake world. Both of them waging a battle that would be seen by none, and would probably be never spoken of.
The final fight isn't one meant to blow your mind with pure baddassery or to leave you in total awe, it's a fight to show you that despite the god's mighty powers, despite the scholar's unrivaled intelect, despite the neverending ability and fame of all the gifted knights and warriors out there, the ones that truly made it to the end, the ones that truly surpassed all the hardships and enemies that stood in their way, were none other than two random hollows, both unknown and nameless to the rest of the world, who didn't count with might, intellect or fame. Instead, they had the perseverance to never give up and keep getting back up to fight, no matter how many times or how hard they got kicked back down to the ground.
Gael suffered through endless harships and battles, constantly losing in the same way we did, yet jumped right back up and kept fighting until he won, just like us the players, before finally reaching the end of the line. Some may see him as anticlimatic, but I find it very meaningful that the last boss of the franchise, and the one to bring closure to the endless cycle of the firelinking curse, is none other than Gael.