What's the reason גברת doesn't work in this sentence?
Well, גְּבֶ֫רֶת means "madam" and should go together with "mister" אָדוֹן. But Israeli society nowadays tends to use more informal ways of address.
By the way: גבירותי ורבותי
"ladies and gentlemen"
I suppose in order to avoid the sound of אֲדוֹנָי, which is already claimed by a certain entity, it is replaced by רַבּוֹת (masters) in the expression "ladies and gentlemen".
YOU CAN SAY GEVER BUT NOT איש? WHY NOT
It should be correct to say איש, and is in fact the way most Israelis would say it.
especially because איש is parallel to אישה.