Garry's Mod 13 not mounting Half-Life: Source

Ever since I have downloaded Garry’s Mod again for 2 years, I have realized I don’t have any Half-Life 1: Source models/NPC’s. Okay, so I decided to go to the mount file options or whatever it is called. Nope, can’t do it. It’s a darkened box with a red X on it. Is there a way I can fix this? I would also like to know why it can’t be mounted ever since the update.

Is your Half-Life 1:Source downloaded? If so,Is it in your steam library. It cannot be a cracked version

HL:S has not been converted by Valve to the new system; nothing Garry can do about it.