This was a question I was asked earlier today. Someone thought it was an exotic digital mode. It is not.
GDX means "best DX in the UK" , nothing more.
If you live in the UK it is probably a phrase you know. If you live outside the UK then it may be a phrase you've maybe never heard of before or were too embarrassed to ask.
In the same way ODX means "best overall DX".
UPDATE 1545z: Just 6m locals and GDX this afternoon. No Es here since CN8LI lunchtime. May be more teatime?