Gates wrote the interpreter along with Paul Allen and Monte Davidoff... while he distinguished himself as a programmer, he's not quite that level of amazing. I get the impression that in a situation similar to (but not as lopsided as) Wozniak and Jobs' arrangement, Allen did a bit more of the technical heavy lifting and Gates planned and arranged the business side.
I'd recommend you read http://www.joelonsoftware.com/items/2006/06/16.html
wherein Joel Spolsky was reviewed by Bill Gates. It is pretty clear the man had serious technical chops.
Way more impressive than sticking together a few perlscripts for a social networking website. Zuckerberg wasn't the main programmer behind Facebook, btw.