is it legal, if I produce something like GPSV9 for you DIYers?
I don't think anyone here is a lawyer so you're unlikely to get an actual answer to that question. And AFIK no one here owns the GPSV9 design so no one can give you permission anyway.
Personally, I wouldn't copy the V9 design itself. The feature everyone needs is a GPS/Mag combo that only uses one connector. The NAZA one fills that need and so would a GPS that has a M8 type gps module but is firmware compatible with the V9. Maybe a higher gain antenna patch and a better LNA and SAW filter.
Or one with a helical antenna. f5soh is doing some experiments with a homebuilt antenna a little bigger than the FPV ones that show that they work better than the patch ones. A company named Sarantel made a really nice small one but they went out of business and all the other ones seem to be more expensive. I'd have thought some Chinese company would be producing something similar but haven't found one.
In short, I'd suggest making something different and hopefully improved from the V9 but firmware compatible with it.