It seems like gratise is not a word at all in dutch. The only reason I can think this may be the case is due to what i read from the tips and rules.
"As with any Dutch grammar rule, there are exceptions. Materials don't get -e, but -en. For example: hout (wood) becomes houten (wooden). Except materials which are loan words from other languages. Plastic remains plastic, for instance; it does not become plasticen."
so maybe "gratis" is a loan word?
It is originally the ablative plural of the Latin noun gratia = thank and means with thanks. (The idea being that thanks are the only payment.) It doesn't really make sense to stick a Germanic inflection on the end of an inflected Latin word. Of course over time the original Latin connection gets lost and such words can be inflected as if they were part of the host language. E.g., in English it would be only slightly weird to say "I have two cum grano salises" as the phrase has already gone part of the way.
As Dutch is losing its adjective inflections anyway, it's unlikely that gratis will ever get one.
I don't know I just looked at your comment and read the tips & rules they said article "De" gets -e that means all plurals because any plural gets the article "De" but here its different, maybe it is just an exception (They said there ARE a lot of exceptions in dutch). if you found a proper explanation please inform me