The NC$$ rule book says no goalie shall exercise the privilege of captain.
Captains wear a "C".  Thus, since there is nothing about players wearing an
"A", nothing in the rule book directly prevents a goalie from wearing an "A".
Gilmore can wear an "A" but he cannot act like a captain.
 
At any rate, since the rule book says that a team must designate ONE captain
for a game and he shall wear the "C" and have all such privileges, the wearing
of the "A" by any player - goalie or not - is for the most part an honorary
thing.  That is what I would presume MSU intends by putting an "A" on
Gilmore's jersey.  It is true that prior to the game, the officials need to be
advised of the captains and alternate(s), but no goalie wearing an "A" can
fulfill the need for a captain, so I'd guess they had another player wearing
an "A" and he was the designated alternate.
 
If either MSU or MTU's captain was unable to play, they could not have Gilmore
or Sargeant step in as captain.  And if their captains were injured or kicked
out of a game, they would have to name a skater to step in, not the goalie.
 
 
- mike