I've tried getting Gentoo to work with Directadmin and it still has library errors. The coders aren't willing to spend any time fixing their broken code, so you might want to rethink using directadmin. There are a lot of other control panels out there.
Of course on the other side of the fence.... The coders are not interested in spreading themselves too thin in an effort to support every known operating system. They are also not interested in turning Directadmin into a big jumbled mess by trying to include everything. Nor are they interested in locking you into waiting on them to post patches that break things because they have mucked up the system so badly.
You already mentioned it yourself .. Gentoo is not in the recommended OS List.
Why dont You consider using a OS which is supported by DA before complaining about it.