I think originally the idea, or rumor at least, was that if you left/ got kicked they'd be removed from you and put in the gbank. But that's not really BoA, that's more Bind to Guild, and then how do you decide who gets to use them? So it's either an idea they scrapped or just a rumor.
They are account bound and therefore once bought are able to be used on any character on that server including those that are guildless or in a different guild. I'm sure if they meant for them to be guild bound they would have made it so instead of account bound. Keep in mind not all guilds allow alts in the guild.
Thats why certain guild rep lvls are needed to be able to buy the gear in the first place.
Once you have contributed to the guild enough that those rep lvls allow you to buy the gear, the said gear is then yours for the rest of its "life". Whether in the guild you obtained it from or now elsewhere