Apart from the UI around the word parsing pop-up being kind of buggy (it doesn't close readily, the scroll position seems to get saved differently depending on the word), I'm pretty sure there's an analytical error in that sentence.
The word "nostra" is glossed as Nom Plur Neut; that is, the possessive adjective "noster", "ours", in its nominative plural neuter form. "Nostra", with a short final -a, is indeed the nominative plural neuter form of that possessive adjective. However in that sentence, the word is being used as an ablative singular feminine, with a long final -â (which is unmarked because the text as they present it doesn't seem to mark vowel length at all).
archive.org has this (https://archive.org/details/caesarscommentar07caes/page/n11/...) 1918 out-of-copyright literal word-by-word translation of the text, which does mark vowel length, and you can see that the translator words the last clause as "the third, (those) who in (the) language of themselves are called Celtae, in ours, Gauls". nostrâ is modifying an implied linguâ there, which is a singular feminine noun in the ablative case, so nostrâ has to inflect to agree with it - nostrâ [linguâ] translates to "in ours [our language]".
My Latin is self-taught and mediocre at best, but even I was able to spot that error. And The Gallic War is a very famous text that is routinely taught fairly early on in traditional Latin education - a lot of students of Latin over the centuries have read that exact line of Caesar's. The fact that the LatinCy neural language model for Latin, which the site claims performed the analysis, screwed this up doesn't leave me very confident about the quality of the grammatical analysis in more difficult corners of Latin grammar.