Some tests found in ukanDu.gfs, ukanZaio.gfs and ukanDio.gfs. I test the forms against Apertium morphological analyser, you can use your favourite Basque morphological analyser.

In this directory I run it like this:

gf --run < ukanDio.gfs > /tmp/Dio.txt

And in the directory with Apertium, I run this:

cat /tmp/Dio.txt | lt-proc -w eus.automorf.bin | egrep -o "\*([a-zñ]*)\>" | sort -u
*didagu$
...
*zatzaie$
*zintzaizkizuekete$

The output is forms that the GF grammar generates but the Apertium analyser doesn't recognise. Some of them are due to overgeneration: forms like "I+me", "you+you" don't exist, you should use reflexive instead, which takes 3rd person object agreement and a special pronoun and all that stuff. So didagu is wrong in the sense that it doesn't exist, but there is no other form that would be more correct. For that reason, I decided to keep these forms and not replace them with an empty string or nonExist. If an application grammarian needs to say "I see myself", they can just use ReflVP in VerbEus.

Some of the non-existing forms are genuine mistakes, like zatzaie, which should be zatzazkie instead. Then you just go to AditzTrinkoak.gf and fix the relevant functions.