What is a 'Sesh'?
According to Wikipedia...
Sesh is a colloquial or slang short form of session. Generally, sesh refers to a period of time spent engaged in some group activity. Increasingly, it has come to mean, particularly among young people, an informal, often impromptu, get-together or meeting to perform a group activity, usually with an element of excitement or air of conviviality. Although a sesh may or may not have a defined goal to accomplish, it generally encompasses an implicit goal of a social bonding experience.
And therefore a Games Sesh is a get together with your friends to play a game of some sort ... well that's what we call it anyway ;>)