Here's how you do it, you have to have access to an account/character with promote/demote privileges to become the GM. In order to kick the GM, you have to have kick permission.
Here's how it's done. You log into an alt with the permission you need for what you want to accomplish. If you want to become the GM, type /gpromote "yourmain'sname" (note: the character you do this to has to be offline) and spam it a few times for good measure. It should say (name)'s rank is too high.
To kick your guild master, just wait til he's offline and then type /gremove "playername". It will say player's rank is too high.
How these work is they both give the usual error messages, but when the character it was done to logs in, they will either be GM or be kicked, depending on what you did. A note about becoming GM though, your rank that you were before you logged in stays the same on the guild tab, but you have access to everything a GM does, including /gleader and /gdisband.
Have Fun.