There are 2 different kinds of outputs here. A license that restricted or applied GPL to the object code emitted by the compiler would create gigantic waves.

There are 2 different kinds of outputs here. A license that restricted or applied GPL to the object code emitted by the compiler would create gigantic waves. The compiler's AST isn't usually an output. This does seem like a dated issue, 20 years ago, there weren't as many C and C++ parsers, now there are a number of free ones. I feel pretty confident that 20 years ago, there would have been some proprietary compilers derived from GCC if they could emit the AST in an unrestricted sort of way. On more than a couple embedded projects I worked on some vendor distributed an absolutely PoS compiler because they had to to sell their parts and couldn't or wouldn't play ball with GCC. Honestly, now I'd think GCC's real value is in the optimizer, not so much the parser. Seems like a fairly simple addendum to the license could GPL the GCC intermediate representation and their usage outside of GCC. That does create a logical loophole in that you don't want code compiled with GCC to be considered intermediate output and then GPLed by definition.