Okay, so in welsh prepositions conjugate to fit the person. "Gan" is a preposition which roughly translates as "with". Using "gan" as a way of saying have is mostly northern welsh I believe.
Mae gen i (I have)
Mae ganddot ti (You have)
Mae ganddi hi (She has)
Mae ganddo fo (He has)
Mae gynnon ni (We have)
Mae ganddyn nhw (They have)
Mae ganddoch chi (You have) (Formal and plural)
These all come in different variants so you might see other variants. I personally use the more traditional "Mae gynna(f) i" for "I have" and "Mae gynnom ni" for "We have", but you're better off learning the more modern forms.
It's a conjugated form of gan "with", here conjugated for hi "she", so ganddi (sometimes more fully ganddi hi) is "with her".
Together with mae "there is", this forms one way to express possession -- mae ganddi, literally "there is with her", means "she has".
You can find other inflected forms of gan here: https://en.wiktionary.org/wiki/gan#Etymology_1_2 (open up the "inflection" section by clicking on it).
I should have realized that was what was going on. Declined prepositions play a principal role from the earliest lessons of the Irish program (and I am familiar with something similar in Hungarian), but they have played so little a role in the Welsh program that I had forgotten about them entirely.
I wrote: "Does he have blonde hair?", but Duolingo marked it as wrong (correct answer: "... blond hair").
But "blonde" is the correct spelling in B.E.
They are simply French words that some English speakers continue to gender as if they were French, much as Americans still pronounce garage as the French do, while Englishmen pronounce restaurant as the French do (or at least some Englishmen). Some English nouns referring to people, such as actor/actress and master/mistress, do have gendered forms, though feminists now often prefer only to use the masculine form. Most such people are still comfortable with gendered words referring to animals, such as rooster/hen, boar/sow, buck/doe, and bull/cow. Of course, we have completely naturalized the French words referring to the meat: poultry, pork, venison, and beef.