Using GB is kind of worthless atm, because noone needs the old materials anymore, because skilling professions got useless.
Maybe it will get interesting again, once flying gets enabled and primal spirits get tradable. ATM there are not many things you can use herbs and ores for, that are not connected to some kind of daily cooldown. Noone needs big ammounts of ores, because the only thing you can do with them is creating crappy items or using them for truesteelbars or taladite cristals, which are pretty limited by the cooldown. For using up the cooldowns, the garrison offers enough ores and herbs.
And another thing. I habe been using the same public profile with GB in MOP since the release of MOP, without getting banned. The account I used, leveled straight up to 90 and did nothing else than farming ore and herbs using the same profile, almost every day. The key to not get banned is to limit the number of nodes per day.
2 guildbuddys got banned for gathering ores, but they were not botting. Why did that happen? When the skygolem came out, they farmed ores like hell by hand for several hours a day because they new, that the price will pop up as soon as it is released. Blizz uses a filter, that basically checks how much you have gathered, how often you have killed a certain mob, how often you have looten a certain item. I don`t know in which period of time, but thats how they autoban you. Thats the most simple and effective way.
The other bans happen by player reports and investigation of a GM.
Using GB for 6h is kind of dangerous. But using GB on 2 accounts for 3h a day seems ok. I would get 2 accounts, farm 1.5h on one acc, than switch to the other for 1.5h, than have a break and start over again. You need to find it out yourself if it is profitable. If you have buyers for the stuff, go for it. If not, don`t eaven start it.
There are some old ores, that work becuase people need them for certain things, but I won`t tell you all the secrets :-D Find them out yourself.