Short and Sweet:
* G.M.T.A. (This is the most common and straightforward option.)
* GMTA (This omits the periods for brevity.)
* Gr.M.T.A. (This emphasizes the "Great" part.)
More Creative:
* Think alike (This works if the context is clear.)
* Like minds (This focuses on the shared thought process.)
* Twin minds (This emphasizes the similarity.)
Ultimately, the best shorthand depends on your personal preference and the context in which you are using it.