Actually I would say that most people do find it euphoric their first few times taking it, it's just that it starts to loose its magic pretty quick which is why it isn't as popular as a recreational drug. A quick look at erowid will show you that people have had some pretty awesome times on gabapentin.
I think 600 mg would be a tease for you and would only lessen the magic your next time actually taking a legit dose. At 600mg you wouldn't appreciate what gabapentin has to offer. With something like gabapentin, I don't see why there would be a need for a "tester" dose like 600mg. Even at like 2400 mg, the effects are relatively subtle. I used to be huge into gabapentin, I guess I responded better to it than most. My standard dose would be 3000 mg. When I first started I would take like 900 mg and it was okay but once I upped it to 1800mg + it got way better.
If you actually wan't to use it recreationaly and have a full blown gabapentin trip, 1800 mg should be enough with no tolerance. Read some of the reports on erowid to get a feel for dosages and what to expect. If you're just looking to supplement other drugs with it you wouldn't need as much. I would highly recommend trying gabapentin on its own though, especially your first few times. For me it was like a cross between a benzo, weed, coke, and mdma, just a bit more subtle, obviously, but still powerful aand euphoric. It's the perfect drug for when you want to feel GOOD in whatever it is you may be doing, but don't want to resort to harder drugs.
Alcohol and benzos muddy the trip and you end up getting more sloshed than euphoric. I would say go with 2400 mg, but no need to jack up your tolerance right off the bat, 1800 is a good amount. Keep in mind, the biggest side effect or recreational doses is a "stoning" effect where your eyes get low and glazed over, and your mental capacity plummits. Vocabulary recall is a bitch and you might sound a bit fucked up in front of others, so don't take more than 600 before work or perhaps family functions. I would consider it much less risky than recreational benzo doses though in that even in high doses you still pretty much stay in control for the most part, just a bit uninhibited, like drinking a few beers. Gabapentin is not something you can really have a bad trip or od on so no need to really stress about taking supratherapeutic doses. Like what was already said, you can only absorb 4 - 5 grams at most