Your Steam Overlay is messed up. Try to enable it. STEAM > Settings > In-Game > Enable the Steam Overlay while in-game.
Hm. Try running Steam as Administrator. I get that a lot too from Planetside 2 since it has a game launcher. It shouldn't do that. If not than try restarting Garry's Mod or try referencing from other people who has the same issues as well.
If you can't get it to work in-game, here is the link. https://www.paypal.com/cgi-bin/webs...bn=PP-DonationsBF:btn_donate_SM.gif:NonHosted Enter the exact amount for the VIP rank you want. VIP details can be found here. Note: This way may be not automatic, so PM @Bayrock or start a thread in the Payments section. Please follow the stickied template when you post or PM.