Garry's Mod dead?

Garry's Mod has been on a steady decline for the last couple years. Youtubers aren't really making videos on gmod anymore, and that's primarily what brought in new players. So I would say yes, gmod is %100 dying, but it's not dead yet and still has a fairly large amount of people playing it considering it's an old game. New servers/communities pop up daily and the niche gamemodes are thriving.

As for your second portion of your post. TTT and Prop Hunt are easily some of the most played gamemodes alongside darkrp. The problem with errors messages is more then likely not the servers/gamemodes fault. Most servers require you to download there content to be able to see everything without broken purple textures or random error messages. Now, TTT servers and Prop Hunt servers have addressed this problem by adding some sort of FastDL, but this still isn't a solution to the problem, and more often then not you will need to download the servers content to remove all broken things and issues.

As for the VOIP. The voice chat on TTT and Prop Hunt is notoriously terrible. This is something players have just learned to accept. Yes it's annoying and yes it's awful, but it is what it is.