Marzooker: Are there any games on GOG.com that are like Diablo 1?
Licurg: Nox, Divine Divinity, and Sacred .
It's important to note, though these all share similar mechanics, they're nothing like Diablo in feel. Nox is one of my favorite, very underrated games, and I highly recommend it to anyone. But despite the many, many comparisons to Diablo, it's not really anything like it aside from some superficial mechanics.
As for Torchlight, this was made by much of the team that made Diablo/Diablo 2 (nope, none of those guys work for Blizzard or worked on Diablo 3, lol). It's way more colorful, but with mods you can actually make it darker and grittier. The storyline is grittier.
Search the Torchlight mod threads in General and the Torchlight sub-forum, there's some must-install mods for it.