I heard an awesome answer in a Dvar Torah given at our shul awhile ago:
The idea starts from the selling of Yosef into slavery: The Yishmaelim who took part in the deal were said to be
"...a caravan of Ishmaelites coming from Gilead, their camels bearing gum, balm, and ladanum to be taken to Egypt." (Genesis 37:25)
Gilad is significant to the story as it was apparently a center for slave-trafficking (unfortunately I don't remember if a source was brought for this part and I couldn't find anything...).
Later on in the Torah, it is said that Yosef lived to see his grandchildren through Machir, son of Menashe:
"...the children of Machir son of Manasseh were likewise born upon Joseph’s knees." (Genesis 50:23)
Who were these descendants of Yosef that were "born upon [his] knees"? One was Machir, who was apparently named after the Mechirah, the sale of Yosef. Another was Gilad, son of Machir and father of Chefer, who was apparently named after the city of Gilad. These two, among the other descendants of Yosef, grew up hearing his story of the sale, his life's story. If Gilad wasn't a center for slave-trade, perhaps he wouldn't have been sold into slavery and wouldn't have had to endure all of those hardships. At his deathbed, Yosef told them not to forget what evil came from the Gilad and to make sure to rectify it. Machir and Gilad received their names so that the Tribe would never forget.
That's why Menashe's descendants go out and capture the Gilad without prompt or permission from anyone - it was always a given that this was a personal mission for the Tribe of Menashe that had to be accomplished:
"The descendants of Machir son of Manasseh went to Gilead and captured it, dispossessing the Amorites who were there." (Numbers 32:39)
and that's why Moshe readily gave them the land:
"So Moses gave Gilead to Machir son of Manasseh, and he settled there." (Numbers 32:40)
Having fulfilled Yosef's dying wish and clearing out the evils of the Gilad, they proceeded to settle there.