"You are not allowed to release your contribution under GPL if you used source taken from the ROOL initiative, then you would have to release it under castles licence. If our code is completly new and has nothing to do with the sources from castle you can release it anyway you want"
I was really talking about the other way round. Imagine if I wrote something useful for RISC OS and wanted to contribute it to the RISC OS Open project. I would have to license it under a compatible license (probably not the GPL in this case), and that would reduce the number of rights users have if they receive the software as part of RISC OS Open.
Of course, if the code is entirely mine, I could also release it under the GPL, but RISC OS Open users wouldn't necessarily be able to take advantage of any improvements that are contributed to the code under a GPL license. Contributors would also have to dual license their contributions. Some may want to, others may not.