I have heard that phenibut might be able to hit GABA-A at very high dosages, but I think it's speculation. Wiki doesn't mention that it has any GABA-A affinity at all.
Phenibut is definitely addictive and I definitely do not recommend more than once every 4 days, at that rate you will not get addicted. I have heard a variety of withdrawal experiences. Some people say it's the worst withdrawal they've had. For me, I have withdrawn from it 4 times in my life, and I find it easier to deal with than any other serious dependency I've had, due to its long half life. Even when I was dosing it daily, I didn't really start to feel withdrawals until 2 full days off, I did feel anxious the second half of the second day, though. So I could immediately start using it every other day instead of every day. Then when I felt stable on that, I can cut out another day, and feel anxious but no severe symptoms on the 3rd day. Basically my strategy is cut out a day, get comfortable, cut out another day, repeat that process until dosing once every 5 or 6 days, and at that point I can just stop and only really experience residual insomnia for a week or two. I have never done cold turkey, and I imagine that cold turkey would get really bad.
However I have read of others having a much harder time. The thing for me with phenibut is, it is not compulsive or moreish. It takes so long to come on that there isn't that instant gratification you get from dosing like there is with, say, GHB (which is relatively similar in effects, as a GABA-B agonist). GHB was WAY worse to come off of, not only because the withdrawal sets in very quickly, but because GHB is so moreish. I can have a tub of phenibut sitting right next to me at my desk while I'm tapering off and I won't take it until my scheduled dosing time. Basically every other addictive drug I've had problems with, the biggest struggle is not caving in and dosing and breaking my taper plan.