I already own a game called Europa 1400: The Guild that sounds very similar to this one. Are they the same game, possibly with/without some of the extras? I'm in the USA. Thanks.

This is the same game, but includes an expansion pack (the main menu screen actually shows "Europa 1400: Expansion Pack").
The expansion pack includes a few new occupations (churchyard warden, entertainer, and tailor), some new titles and laws, a new town, and a few other small enhancements.
It is still not clear why this appears on GOG as "The Guild", but appears everywhere else as "Europa 1400: The Guild"... although I have to admit that GOG's version is catchier.

I think in europe it was sold as "The Guild". At least here in germany where it was made it was called this way (strictly speaking "Die Gilde", which means exactly this in german) and i can imagine that they just translated it. :)
Why they changed the name, supposedly for the US of A market, i don't know. Maybe they thought that the average-american guy doesn't know what a "Guild" is but at least knows where europe lies and that the year 1400 did take place here some time ago and that the people back then surely did some sort of trading? ;D