Don't you have to do a bunch of complex crap to get add ons for G-mod? Way more complex than Blockland? Tons of steam players play Gmod, I'm pretty sure the fact that you have to go into the games files to install add-ons is not a problem.
The only problems that Blockland would have with Steam are things Badspot knows more about than we do. Maybe he would be able to update the game fine if it was with steam, maybe it'd be extremely hard for him. Really only he would know.