Everybody's different, and there is no single way to combat GAS. I found that it helps to have goals and to keep notes.
The problem I've had with buying stuff because it "completes a set" or because "it's a bargain" or for various other non-musical reasons is that I'd end up not using this new gear for more than a few days, weeks or months. After that, the "cool" would wear off and the gear would go neglected. But it'd still be there and I'd feel guilty about not playing it, so I'd drag each piece out for fifteen or twenty minutes to compare against another bit of gear or to experiment with setup or settings or whatever... The point being that I had a bunch of gear that didn't contribute at all to making music. In fact, just having this gear distracted me from making music.
It got pretty bad. I had a large closet stacked full of unused gear and our living room was starting to look like a well-stocked music store.
I started to turn this around by selling lots of gear that hadn't been out of storage more than a few days in the past year to a local used-gear shop. I knew their markup policy, so I had reasonable expectations regarding how much I'd get for my gear. They spent a day doing research and pricing and called with a price that was within a hundred dollars of what I expected. We talked, and they wrote me a check for the figure I had in mind. Yes, I took a hit by not selling the gear myself, but this was my "bottom of the barrel" gear. It would have taken me forever to sell it one piece at a time (we're talking about enough gear to fill the back of a minivan with the seats taken out), and I would have had to deal with lots of strangers traipsing through my home. For the peace of mind alone, it was worth selling the gear to the store.
I sold the nicer, more expensive pieces according to the same rule: if it's been sitting mostly idle for the past year, it's gone. I've followed that rule ever since; it keeps me from accumulating too much stuff, and the discipline of evaluating each purchase after the "honeymoon" period has sensitized me to the risks of making impulse purchases.
So what does all this have to do with having goals? That's simple: I've had to make sure that my gear purchases supported my musical goals. I expect a certain kind of response from my gear. It has taken me a few years and quite a bit of buying and selling to get to the point where I'm truly happy with my gear. The important thing is that I had a goal in mind and that every purchase moved me closer to achieving that goal. For me that involved keeping notes about what worked and what didn't and doing research to learn about how design, construction and material influenced what I heard and felt when I played my instruments. That last point is kind of subtle, but very important; I didn't buy on the basis of my research, but rather did the research to try to understand why things worked (or not) for me. A musical instrument is a very personal means of expression; you don't want to rely on someone else's evaluation if you expect an instrument to meet your expectations.
The tricky part, of course, was being realistic about my goals. I'm no hack when it comes to playing guitar. On the other hand, I'm far from being a studio ace. To put it bluntly, I need to be aware of my limitations and set my goals accordingly. I have a certain style and a certain sound and a limited amount of time to spend on making music. My goals for musical improvement follow as a natural progression from what I already do well. I don't kid myself that I'm suddenly going to take on an entirely different genre or style because I found a piece of gear that would work really well for that. On the other hand, avoiding those kinds of purchases doesn't seriously limit what I can do with my music. My signature tone is mostly-clean, and there's plenty I can do musically within the constraint of that underlying tone. Although it can be fun to play with extreme gain once in a while, that's not my strong suit and I don't feel the need to have a separate rig optimized for that tone for the rare occasion when I might want to give it a try.
Over the past five years or so I've gone from way too much gear (much of which didn't serve any real purpose) to a small number of amps, guitars and pedals that all fit my musical goals. As an unexpected benefit, I have less money tied up in better gear; the extra money sits in a savings account against future purchases. And when I go into a music store I can look without being tempted to walk out with some new impulse purchase, because I now evaluate all potential purchases against my musical goals and existing gear.