This is pretty simple but it needs guidelines.

You cannot off-site advertise as it is against the gaia TOS!

You are allowed to advertise your guilds as long as they in return advertise this guild.

You may show link's to your arena entries and stuff relating to artshops, it can advertise any part of gaia. IF you have sites with virtual pets or something and need them to be clicked here is the area, as long as it doesn't violate TOS it can be done here.