GMA (glycol methacrylate) is a poor embedding media for IHC protocols
in general, since you CANNOT remove the plastic which prevents 
immunoglobulins from reaching antigenic sites.  Some people have success.

You need to refer to Neil Hand's article, in Journal of Histotechnology
on "Using Superheating using pressure cooking: its use and application
in unmasking antigens embedded in  methylmethacrylate" 21:231-236, Sept 1998

He microtomes PMMA (polymethyl methacrylate) embedded tissues, 
has a huge panel of antibodies
(well over a hundred) which he successfully stains.  He removes the 
PMMA before staining, with wonderful results.  He has further references in
the article and can be reached via email, he has tried GMA, plus many other
plastics for IHC. mpznhand@unix.ccc.nottingham.ac.uk.  Give him a jingle  

Gayle Callis