Are you sure you have installed them correctly? Try following those steps -
1. Download the megapack (In parts)
2. Extract each part to the same directory (any directory will do for now). So you will have a "Materials , Models , Sounds ,Maps , etc" in one folder.
3. Take all of those folders and put them into your garrysmod/garrysmod directory.
4. Override if nessesarry.