I don't think the Latin /g/ has anything to do with the French /d/, actually. French generally shows epenthetic /d/ between historic /n/ and /r/ (although there are some exceptions, such as vinrent and tinrent, and some words such as genre that appear to be exceptions). Consider that tenir, which never had a /g/, has the future stem tiendr-. It's the same sort of sound change that gave us "d" in English thunder from Old English þunor.
I don't know for sure, but based on the stem of this verb having /ɲ/ when it's not before "r", I would guess the development from Latin was something like [piŋgɛrɛ] > [peɲɟɛrɛ] > [peɲɛrɛ] > [peɲrɛ] > [pejnrə] > [pejndrə].
A change of Vulgar Latin /ŋɡʲ/ > /nj/ > /ɲ/ is listed on the Wikipedia article "Phonological history of French" under the section "To Proto-Gallo-Ibero-Romance". A later change it lists under "To Early Old French" is "Palatal /ɲ/ [is] depalatalized to /n/ [...] following [sic] a consonant. /ɲ/ > /jn/ when depalatalising". I assume the author intended to write "preceding a consonant."
There is no general change of Latin -ngr- to French -ndr-.
There are words that contain "ngr" in Modern French from Latin "ngr", such as palangre which the CNRTL says is from Latin *panangrum or congre which it says is probably from Latin congrus.