‘ Then the Spirit of Yahweh came on Jephthah, and he passed over Gilead and Manasseh, and passed over Mizpeh of Gilead, and from Mizpeh of Gilead he passed over to the children of Ammon.'
Jephthah was now taken possession of by Yahweh, and he went through Gilead and Manasseh (not necessarily in person) gathering further troops to join those already gathered in Mizpeh (Judges 10:17). Judges 12:2 may also indicate that he sent a summons to the tribal confederacy. Then he reviewed his army at Mizpeh of Gilead and was satisfied. So then he set off with his men and his army to face the Ammonites.
Alternately it may be that the troops that had gathered at Mizpah (Judges 10:17) had returned home to see to their fields and flocks when no leader was forthcoming, and thus had now to be re-gathered.