The Apple//gs open source ecosystem - forever teaching to the future generations

You can buy the sources of the BASIC, C, Pascal compilers from the ByteWorks:
https://juiced.gs/vendor/byteworks/
https://github.com/byteworksinc/ORCA-C
Apple IIGS ORCA/C Compiler, an ANSI C compiler for the 65816 with libraries for the Apple IIGS

You can recompile GS OS 6.0.x with MPW (Macintosh Programmer's Workshop) on the old Macintosh system :
https://archive.org/details/GSOS_6.0.1_source
https://www.youtube.com/watch?v=L1tLhckz5q4
Building the Apple IIgs System Software 6.0.1
  Antoine Vignau
  315 abonnés
  This is a 10 min video showing how to build the Apple IIgs System Software 6.0.1 and its 
  components (from GS/OS, going through the toolbox, ending with the Finder) - Let's go to 6.0.2 now!

You can install GS OS 6.0.2 with these disks :
https://www.callapple.org/software2/appleiigs/gs-os-6-0-2/
Downloads:
2mg Disk Images: https://www.callapple.org/gsos/IIGS System 6.0.2 2mg disks.zip
PO Disk Images : https://www.callapple.org/gsos/IIGS System 6.0.2 PO disks.zip
Other Updates
GS/OS Boot block+loader for HFS : https://github.com/ksherlock/hfs-boot

GNO is the UNIX-like environment for the Apple IIgs 
You can install GNO/ME (Multitasking Environment) with these 18 floppy disks :
http://ftp.gno.org/pub/apple2/gs.specific/gno/base/v206.19990215/
You can recompile the sources of GNO :
https://github.com/ksherlock/gno
https://github.com/ksherlock/gno-kern
https://goldengate.gitlab.io/manual
https://goldengate.gitlab.io/debugger

You can recompile the MERLIN32 open source cross-assembler for the Apple//gs :
https://github.com/apple2accumulator/merlin32
The Merlin 32 assembler for the Apple II from Brutal Deluxe Software. 
More information, documentation, etc available at:
http://www.brutaldeluxe.fr/products/crossdevtools/merlin/

You can recompile the KEGS Apple//gs emulator :
https://github.com/aufflick/kegs
Kegs is an Apple //gs emulator for Mac OS X, Win32, Linux and Unix/X11
http://kegs.sourceforge.net/
Download the KEGS v1.16 tar.gz file (Executables for Mac OS X 10.13+ 
and Linux, sources for Mac and Linux)
GS+ : An Apple IIgs emulator based on KEGS :
https://github.com/digarok/gsplus

CiderPress : A Windows utility for managing Apple II file archives and disk images. 
Visit the web site at http://a2ciderpress.com/.
https://github.com/fadden/ciderpress

BASIC listing programs for the Apple // :
http://www.hebdogiciel.free.fr/apple.htm

Apple //gs public domain pdf books :
http://www.apple-iigs.info/