It's what you say every 5 seconds on a 50's sitcom. Goes great along with that's swell. Means: oh shoot, darn it, wow, that's grand
Can be used in a variety of ways.
Gee wiz, Dad, you sure are swell!
Another phrase to use if you ever appear on a 50's sitcom.
Goes along great with gee wiz!
means: that's great!
Little Billy: Gee, That's swell! I've never ridden a bike before!