For those that weren't around (or didn't do programming back in those days).

The GUS soundcard just filled a niche perfectly at point in time with a reasonable pricepoint.

It came out at the same time as the Sound Blaster 16, but while the SB16 required "expensive" software mixing for anything but prerecorded audio the GUS handled that in hardware meaning that users either traded for CPU performance or audio quality in games.

In the 486 era it was really a differentiator and also caught on in the demoscene, by the time Pentium processors rolled in the software mixing cost became far less pronounced.

Finally when Windows became the go-to for games (and CPU's got even faster, esp when MMX processors came into play) the API's made cheap hardware equally good since most developers just targetted the plain streaming API's (with software mixing) for broader compatibility.

The final holdout for GUS cards afaik was the demoscene, since the card had a built in mixer with frequency control it could play modules (.MOD, .XM,etc) with simple players that just uploaded the samples to ram and then just changed registers realtime (kinda like an Amiga but more powerful) instead of including a mixer, in the end however it was more about code-size than cpu performance, doing a 64kb intro you could shave off something like 3-10kb's compared to supporting a SoundBlaster card (depending on other tools) and those extra KB's was well suited for effects/art.

In the end though, people realized that the dosextender+GUS combo was as heavy in terms of code as just a mixer on Windows (since you didn't need a dosextender) and when better compressors arrived for Windows even the demoscene moved on.