Pretty much everyone doing guild battles are level 20. If you don't have a lvl 20 character, you may want to consider making a PvP char to engage in said battles.
Now that you have a guild hall, you have to join the masses in Sardelac Sanitarium suggesting all the ways that guilds and guild halls need to be improved in the game
Another question... What level are most people that fight GvG? And what happens if I only have say 5 people to do GvG instead of 6.
GvG is done with 8 people, not 6. There is no level restriction either. I guess you can try to do it with 5 people and under lv 20, but you probably wouldn't stand a chance against any decent guild.