Well you are on a pretty small dosage of buprenorphine, though its still pretty weird that Codeine would still be effective. At 2mg, I could break through with 30mg of oxycodone, but no less (and I shot it too). Buprenorphine acts very strangely and uniquely for every individual, especially when it comes to how effective it is at blocking other opiates. For the most part I've found that it blocks 50% at 4mg, though you can get full effects if you wait about 36 hours. 8mg seems to block about 75%, and takes at least 48 hours to be able to get a respectable buzz out of heroin. However, there have been times where I'll do one bag of dope less than 12 hours after dosing 16mg of suboxone (chronic dosing too) and get completely high like I was never on it to begin with..
So if you can feel codeine and other opiates at 1.7mg, I wouldn't be worried too much about it. Most likely you metabolize bupe at a rather fast rate, or perhaps you take other medications that could speed up it's metabolism? In the end, relying on bupe to block other opiates can only keep you clean for so long anyway, learning to resist the urge to use when your receptors are not blocked is more important anyway. (but initially I can see the argument for why it is good for people to be on these blocking doses).
As for your initial question, yes, 300mg of gabapentin can definately get you buzzed. I hear about people starting off with over a gram of the stuff and it blows my mind. 300mg will give you nice anxiolytic effects, as well as a pleasant body high. While it won't directly potentiate codeine, it does synergize nicely with all opiates and it's a great tool for dealing with WD.