If that is the case you should have been given the choice to dethrone the original GM and take the guild. Unless they set the ranks up so that you are far enough down that it doesn’t give you that option.
I have actively been given the option to take control of guilds before. Even as the lowest a low ranking member. However they were guilds that didn’t have stretches of inactivity as long as yours.
In those cases I have:
- Noted the original GM.
- Set the Guild Message of the day to note that I have taken the guild.
- Left my main’s contact info in my char info.
Because I have assumed that the GM “might” want it back. Which I would be happy to do.
Most, when returning, have thanked me. Especially after I have talked to them about how the Auto Dethrone works. Though one guy gkicked me right away after I handed his guild back. 
His guild, his choice.
Blizzard links:
[1] - See the response/reply from @mirasol down thread.
Edit to : Make a correction to bring my explanation into line with the current system now.