When did he give these gifts to them?
Malbim: It was when he returned from Ramos Gil'ad. Melachim II, 1:17 implies like this. It says that Yehoram ben Achav became king in the second year of Yehoram ben Yehoshafat, and afterwards (3:1) it says that it was in Yehoshafat's 18th year! Rather, we must say that Yehoshafat made his son Yehoram king in Yehoshafat's 17th year (when he returned from Ramos Gil'ad).
What are "Migdanos"?
Metzudas Tziyon: This refers to top quality items, be it Peros, garments or gems. This is like "u'Migdanos Nasan l'Achiha" (Bereishis 24:53).
Why does it say that he gave the gold, silver and Migdanos with the cities?
Kidushin (26a): This teaches that Metaltelim are acquired along with land. (A Kinyan on the land acquires also the Metaltelim.)
