Do all other adjectives (not with the verb "to be") have endings for gender?
Predicate adjectives also occur after some other verbs, though "to be" is the most common. For example, "I painted the room blue" or "I became angry".
The other kind is attributive adjectives: ones that are before a noun (or that stand instead of a noun).
Those (nearly always) have an ending which indicates the gender, number, and case of the noun.
Which ending is used depends not only on gender, number, and case, but also on whether there's an article or other determiner in front of the adjective, and if so, what kind -- there are two sets of endings (strong, for when there is no determiner, and weak, for when there is a definite article or similar determiner), as well as a mixed inflection which takes some endings from the strong set and others from the weak set.
For example: heißes Wasser "hot water" has the strong ending -es, while das heiße Wasser "the hot water" has the weak ending -e.
See https://en.wikipedia.org/wiki/German_adjectives for more.
The main exception for adjectives before a noun but without an ending that I can think of is viel "much, a lot of" in the singular -- it's usually viel Wein, viel Limonade, viel Wasser and not vieler Wein, viele Limonade, vieles Wasser. I don't know why. In the plural, it always has an ending: viele Menschen.
Fully agree... The "ist" signals that it can't be "Fliegen." BUT it does sound (in normal speed) a lot more like the female voice is saying "Fliegen" rather than "Fliege" (the male voice is ok, if that's what you get to hear). Duo can be tricky sometimes, but this sort of trickery isn't a Duo habit/practice in my experience (a Duo user for 4+ years). "Ehrjon" 9 May, 2020.