Honestly, if you're in Gastown, you can't miss the Steam Clock. But is it worth waiting the 15-min interval for it to toot its horn? No.
That being said, while the clock itself is a miss (unless you are a fourth-grade kid or a Japanese tourist), Gastown is great. There are some fantastic boutiques along the whole strip (interspersed with the touristy kitsch) and some really great restaurants.
If you didn't have a sheltered suburban childhood, the nearby Downtown Eastside is worth a looksie.