In general a buffer overflow is when a program (such as gmod) is writing into memory called a "buffer", which is basicly memory used while moving data from one place to another (from an input to an output for example), is overflowed and writes into memory outside of the buffer. I don't know how that extends into Garrys mod but I doubt there much you can do to fix it - I get it often and I find rejoining usually helps, or going from singleplayer to multiplayer...