What were the family of B'nei Gil'ad concerned about?
Rashi and Ramban: They were concerned that, should the daughters of T'lofchad (who were from their tribe - Menasheh) marry husbands from another tribe, then, when their husbands die, their sons will inherit them, thereby transferring the portion of Menasheh to another tribe (before they have even settled in the land! 1 ).
Seforno: They were concerned that, if the portion of the B'nos Tz'lofchad would end up in the territory of another tribe, the borders of both Menasheh and the other tribe would be messed up. 2