99% gpu usage is normal for games.
of course your gpu is going to try and go full on. I have a 770 and get the same thing. Games are demanding but everything is normal. Even if it isn’t demanding, unless it’s really lightweight, the GPU will put in it’s 100% to run the game as fast as it can.
Worry when your temps spike to 70c. That’s when you know that, unless it’s dusty, something is going wrong. Even if temps are bad, what you really have to worry, really really bad, is when it goes to 90c and above, that’s when you have to immediately clean the computer and ask some forums. Nvidia can handle 120c but I wouldn’t go above 70c personally.
70c is acceptable but only under heavy stress. You should also worry about older games that run over 120 fps, that’s when you should cap the framerate to ‘lighten the load’ with either vsync or an in-game feature.