You can tap any untapped creature you control, including one you haven’t controlled continuously since the beginning of your most recent turn, to pay the cost of Town Gossipmonger’s activated ability. You must have controlled Town Gossipmonger continuously since the beginning of your most recent turn, however.

For more information on double-faced cards, see the Shadows over Innistrad mechanics article (http://magic.wizards.com/en/articles/archive/feature/shadows-over-innistrad-mechanics).

Incited Rabble’s controller still chooses which player or planeswalker it attacks.

If, during its controller’s declare attackers step, Incited Rabble is tapped or is affected by a spell or ability that says it can’t attack, then it doesn’t attack. If there’s a cost associated with having Incited Rabble attack, its controller isn’t forced to pay that cost, so it doesn’t have to attack in that case either. Note that transforming Town Gossipmonger won’t untap it.