GAS results from two major things: first you have GAS because you have decision problems and lack of experience. you simply dont know what you actually need - just yet. so … almost everything which has good marketing behind it is something you instantly want.
once you gained experience over the years and found out what works for you, you have GAS because you see something that looks promising and might fit into your workflow. and if you GAS really hard for it you suddenly find yourself feeling incomplete without this very piece of kit. its interesting because you dont even know the machine yet, you just saw it, you heard it - but without having it in your hands you already think: you need this. it sounds awesome (although its often edited), i must have it.
i think that GAS is just a natural thing. we are kids in our hearts and every new toy is always exciting. but we have to learn to control it. if we sold enough gear to get new gear we one day realize that its never the gear that makes us a better musician, its us. nothing else! to counter the GAS i therefore always have a look at what i’ve done and with what i have done it. if its comfortable to work with what i have, if it helps me getting the job done, fast - i know from experience that adding new gear not only means that i get new possibilities - it also means more work and headache getting into it and adding it to the established workflow in a meaningful way. since i experienced this several times already i know what might wait for me if i let the GAS take control over my decision making process 
this helps a lot making the right decision in the end.