But to answer my own question, Gad was obviously written into the film so that we'd focus all of our attention on him and not notice Adam Sandler's horrible acting.
"It's not working! Release the Dinklage!"
Why bother, though? Whenever Sandler plays a man-child (like his character in Pixels), he almost always does a piss-poor job with it. Billy Madison, Happy Gilmore, Little Nicky, The Waterboy: some of those movies might be good, but Sandler's performance isn't. But Pixels was especially grating because Adam Sandler is now an old man, so it's more painful when he tries to act like a big kid. Twenty years ago, it was alright for Sandler to pull his "I don't want to grow up. I want to get drunk and cause funny havoc!" shtick. Now, at fifty, whenever he tries it, all you can say is "Yeah, that's cool, but where are the bodies?"
That's why he should stop doing that and only play actual adults from now on, and not just people that are adults in the sense that they have more pubic hair than the rest of the cast. And also because he's really good at it, like in Hotel Transylvania where he voices Count Dracula being overprotective of his daughter.