A) this clearly doesn't belong in wikipedia. if it bleongs anywhere, it belongs in the wiktionary, and i sorta doubt they even want it there. 
 *B) I'm prety sure the term is GrILF 
 *C) you should capitalize all the letters of GMILF in the body as you do in the title, both to retain consistency and because it is an abreviation.  
 *D) if you want to take this project seriously, you should try to relate the history of the term, tracking back to 'MILF' 's first usage in some american pie movie ( i think).  
 *E) as you say, the main usage of this term is slang. i really hope it's not that common as porn code, but hey, i don't go looking for grandma porn.