I don’t think it’s accurate to say Roblox replaced Gary’s Mod. They both released in 2006, and they’ve always targeted different audiences. Roblox has always targeted younger kids while Gary’s Mod has targeted older kids, teenagers, and adults. Roblox has had a weird resurgence in the past few years, but it didn’t replace Gary’s Mod. Heck, I’d reckon it was always more popular than Gary’s Mod among younger kids (I know I had friends who were obsessed with Roblox circa ~2009, but I didn’t hear about Gary’s Mod until I was a teenager).

You're right - I think I could have made a better comparison.

Maybe a better comparison might be Source engine custom games to Minecraft mods?