This question is really broad and can only be answered in a very general form.
Most of the German pronouns are actually flexed (Pronouns stand for nouns, and nouns have to be flexed as well). Some of the flexed forms of pronouns are, however, identical, some don't exist at all. Some pronouns (viel, wenig, ...) are normally used to defer to uncountables or some kind of "anonymous mass" where gender doesn't matter, in this case German falls back to a gender-neutral form, which is normally the male form.
Taking your examples:
Viele werden sterben [male and female forms are identical]
Viel(es) wird geschehen
A word that shows more differences when flexed is manch-
Mancher hat schon sein Schäfchen ins Trockene gebracht
Manche legt sich morgens wieder hin
Manches sollte so eigentlich nicht passieren
Manche legen sich morgens wieder hin [plural identical for m. and f.]
So yes, pronouns are flexed, and, like all nouns in German they (normally) have all three genders - It's just that most plural forms of those pronouns look identical for all three genders.
We have now only looked at gender with pronouns - obviously, the cases are of interest as well and affect the flexing.