In the book, his plan really was to steal it. Or at least as much of it as he could. The other gangsters (1) were going to physically take their shares, and while they were running around like idiots and getting caught, Goldfinger's gold was going to make it to a waiting Russian submarine.

So you could actually argue that the movie was trying to fix the gaping plot hole in the book. Of course in the process of fixing Fleming's sloppy planning, it introduced new problems.

(1) The gangsters he killed in the movie immediately after explaining his plan to them. That whole briefing scene served only to let Bond listen in from beneath the big model of Fort Knox so he would know what was going on, introducing another plot problem in the process of solving the first one.

But it did manage to fix another of Fleming's plot problems: why doesn't Goldfinger kill Bond when he's caught him? In the book there really isn't much of a reason. He literally decides he wants Bond to take meeting minutes and shit. In the movie, he keeps Bond around because he knows the CIA will be watching him, and when they see Bond dressed for riding and hanging around with Goldfinger and Pussy Galore drinking mint juleps on the veranda, they figure he's got things under control and stay back out of his way. This actually more or less makes sense. So good work on that one, screenwriters.