How to (not) use GPL Software
GPL And Embedded Systems

Historical background:
The GPL was written for userspace programs running on existing operating systems
Covering a whole OS (and even userspace programs) is not an ideal match, but if you read it carefully it still makes sense

Toolchain:
"... the source code distributed need not include anything that is normally
distributed (in either source or binary form) with the major components
(compiler, kernel, and so on) of the operating system on which the executable
runs, unless that component itself accompanies the executable."
Practical case:
You've modified gcc for a specific embedded platform
Therefore, this gcc is not "normally distributed with the operating system" and you have to distribute it together with the source code
gcc itself is covered under GPL, so you need to provide binaries and source code(!)