Spring Roo by default uses flexJSON
which is probably not suitable for some issues.
Is there anyway to use GSON
instead (expect the hard coding part) ?
There is no easy way to do it.
Now, I can see to ways to archive it:
Depending on how many classes you need change (should be too many classes to worth it), the second option could be a good approach (also, then you can share it ;-) ). You can start from Roo add-on documentation.
Good luck!