Just tried it yesterday on my guild's GM, a friend of mine as a prank. By using
/gremove (player name goes here)
it will say "That players rank is too high". However if that person logs on he will no longer be in a guild even though it says he is still in the guild on your screen. This causes many bugs, as a new GM is not designated. Also if you try to reinvite the person it won't add them into the guild immediately. You need to reinvite them and then they have to gquit and then you reinvite them once more putting them at the lowest rank. In order to get a GM for the guild you have to put up a ticket requesting that a GM be designated for your guild which took me about 2 hours to get a response

. Just wanted to post some more information on this matter as it doesn't seem crystal clear in the first post.