In any kind of creator or sandbox game, you're going to have to deal with some lag if you summon enough objects, depending also on how detailed they are. Even LittleBigPlanet 2 you can make the game freeze or severely lag by spawning enough junk all at once.
Only time I remember lagging in Gmod was when I tried to spawn a VERY large and highly detailed Battlecruiser model from Starcraft 2.
*edit*
Also for anyone who might not know yet, you can download a ton of ripped models to use in Gmod from here
http://www.garrysmod.org/downloads/The site hooks up directly to your Steam account when you register if I remember correctly.