I would definitely not substitute GHB for alcohol when on baclofen. GHB is not at all like alcohol, well, maybe a bit similar but nothing has made me say "ah fuck it, who cares about my dosing protocol, I'm sure I can handle a little more" quite like GHB. Passed myself out a number of times like this. I'm really bad when it comes to GABA-ergics though. I also managed to black myself out of several days on Etizolam, even one evening spilling my stash on the floor when trying to redose the 5th time (3 times too many at minimum) and literally soaking the liquid on the floor with a kleenex and parachuting it (dose unknown) to another lost 20 hours.
That said, I think more than just myself can have a problem moderating doses of GHB - it is nowhere near as easy as not drinking another beer (which I am firmly able to do).
I also have had Baclofen. Relatively easy to take a dose and call it quits, but then I figured it would be great to go with Pregabalin, both impacting GABA in different ways, why not try and get the full spectrum and all. Definitely had an issue continuing to take little bits of both over the course of the evening, even when I was clearly subjectively FUCKED UP. Girlfriend came home to me acting like a completely different person (I don't remember this) - just weird somehow, she instantly knew I was messed but didn't really say anything. Then we went to bed and I passed out breathing weirdly with my arm spasming like wild and she tried everything she could do to wake me up. She would have called an ambulance if she hadn't interacted with me before I went to bed and she was more worried more than angry (she said she was sure I would sleep through a fire and she wouldn't be able to wake me up or carry me out in that state). I promptly threw out my Baclofen. We have a bit of a "don't ask - don't be a drugged the fuck up nightmare to deal with" policy on my habits. I invariably mess up every now and then but that was definitely one of the worst.
Pregabalin is pretty nuts in high dosages and kind of comparable to 1/3rd of the feeling of GHB but it has a bit of a ceiling effect. I worry that combining Baclofen with GHB would be a respiratory depression nightmare and you'd feel so good until you took just too much. Obviously you are going to do what you want but that combo scares the hell out of me.
Side note, while GHB has the capacity to be a horrible nightmare all on its own (so don't take this so much as a recommendation, just more like harm reduction), if you are dead set on using it, why not try it on its own, I'm sure it will reduce your muscle spasms to a reasonable degree or at least make you not care about them. Or at least only use GHB on weekends and not take your Baclofen that day.
Final note (I'll stop, you don't need a book of my ramblings), GHB is alcohol-like but it is not alcohol, it is way more fucking fun. Even if you successfully don't kill yourself with this combo - you are going to push it with the GHB and you are going to either be late for work because you passed out before setting alarms and oh hey, its 11am or you are going to wake up, still be a bit wasted from the night and drive dangerously to work where you will probably be at least a little haggard. It's very hard to do "a couple drinks worth", even if you measure it out and put the rest away for the night. After even a couple of days or a week, you will take your two ml, realize it isn't doing what it used to, and up the dose to 4ml or 2X2ml, one of the problems with it is that your enjoyment tolerance scales differently to the safety tolerance and you can get to a point where the amounts to get one are dangerously close to when you succumb to the other.
Cheers and be safe