Arduino (the original AVR boards anyway) have always relied on GCC, and not just that but the entire open source chain that already existed for AVR-GCC. I'm sure they contribute back (I guess "sure" is an exaggeration), but it worked pretty darn well already.

Arduino, for me, replaced emacs for an IDE. The main reasons I use it are because I don't need to write a makefile, and the integrated serial port. Those are good enough features that I still use the IDE even though I haven't touched a real Arduino in a decade or more. But I work alone and don't usually have more than a few thousand lines of code so it's not too complex to manage.