If for some OCD style psychopathic reason you subscribed to all of the mods including those from the linked collections, garry's mod can't handle that many items from the workshop. The engine can handle up to 8k extra lua scripts but steams buggy workshop feature can't handle more then a handful of mods. The only thing you can do is unsubscribe from each mod. As this is about 2100 mods, that could be a headache. Until steam includes a unsubscribe from all feature... Quick way is to login to garrys mod workshop via chrome, go to subscribed addons check 30 mods per page and hit f12 and input this in chrome's console *hit enter each time* for every page until they are all clear, good luck:

(function(){jQuery('.btn_grey_black').map(function(){this.click()})})()

Basically runs a javascript code that unchecks every subscribe button for you at once on a page.