If it is possible not to depend on JDK, that will be preferable. There is a working OpenJDK 7 for PowerPC, but it may not have some components. OpenJDK 8 at the moment does not build.
As for GCC, the latest versions are working fine on 10.5 and 10.6.
Cocoa may not be properly supported on PowerPC, as well as Obj C stuff (i.e. may or may not work).
If it is possible not to depend on JDK, that will be preferable. There is a working OpenJDK 7 for PowerPC, but it may not have some components. OpenJDK 8 at the moment does not build. As for GCC, the latest versions are working fine on 10.5 and 10.6. Cocoa may not be properly supported on PowerPC, as well as Obj C stuff (i.e. may or may not work).