<Random rant>
Open source or not open source the GP32 development community is generally very good at helping one another out with projects, code, concepts etc. if you ask nicely

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Personally I see open source as away of inviting collaborative development without being picky about who does said development

, any community development is a good thing and just writing code is a part of development not the be all and end all. There are many other tasks then just writing code that can help.
Open source will mean that a project does not necessarily die when development ceases and if nothing more, parts of the project can live on in other derived works and that IMHO is a good thing.
If I was to use any GPL/open source code in my apps then I would feel bound by the appropriate licence and release my code under said licence if I was to release a binary (important point, most open source licences DO NOT require you to share you code just because you have edited/tweaked it, but only if you publicly release a binary compiled from said code).
If I write my own code from scratch then it is entirely up to me if I release it open source and that is my decision alone, same applies for any other developer, it’s there work after all and they can do with it as they please as long as you respect the wishes of those who (or who’s code) help you.
As for tweaking other people’s code, yes it is done, quite often infact. A lot of GP32 projects have had several ‘maintainers’ in there life span.
</Random rant>