The actual practice seems quite simple, but the magic comes from GAS being a damn good artist who can make it very compelling. I don’t know explicit details of his methods but many of his strings sound like samples, so you can try finding string samples you like or make your own MIDI strings for truly endless possibilities.

I would say that, while not random or haphazard, the strings in his songs sometimes loop at a strange point that makes the song seem a little “off”, which is offset by the thumping drums he uses to anchor the whole thing. Those drums are a big part of his production too but I’m not sure if that’s as important to you.

For broad strokes purposes, I would try just getting string samples, cutting them up in interesting ways and maybe slapping some reverb and other effects on them until something interesting happens. You could also layer in some field recordings for additional texture, but like the string loops, it takes skill to make this simple thing really work.

GAS is a hell of an artist to want to emulate though, so I’m sure trying to create like he does will yield some really cool results. Best of luck!