i have been getting a few recently, one being this:

also, another one which i havent been able to screenshot, it says something like it ran out of memory? and telling a programmer to up vertex buffer size? anyway, what do these mean and how can i fix them? - thanks

Try re-installing Garry’s Mod.

On a side-note, there is a help sub-forum.

i have had these problems before and after restalls, thanks anyway

The DX8 error is to do with DirectX which is your graphics card. Do you have the latest drivers for your graphics card?