Future Gohan is in alternated future where Goku dies when Frieza is a robot a Trunks doesn't kills Frieza and King Cold. When in the future Future gohan and trains
trunks(without sword and not yet in ssj1) When dr.gero gets to his lair he turns on 17,18 . When future gohan meets the 17,18 he battles them to the death . When gohan rembers everyone of the z fighters deaths. He then goes ssj1 fights 17,18. He loses his left arm and trunks comes and tries to help out as much as he can.Gohan then goes ssj2 when trunks is nearly dead. gohan uses the super kamehameha.
Both 17,18 surivive and head up in the sky and shoot at gohan with yellow beams like a machine gun and gohan cried I'm sorry dad and trunks I failed you all now.....your dead I wish i had stoped them all. Gohan lyied dead in a pile of ruble in the half destroy Trunks reaches for the bag with a siesnuz bean and eats it and goes ssj2 and kills the androids he crys with tears of sorrow wishing he can change the future . For 4 years and builds a time machine with his mother and his grandfather. Goes back in time and kills robot frieza and king cold and tells goku that he saved his life. Trunks tells goku,gohan,piccolo,vegeta,yachma,teian,chaizoto,bulma . that trunks;s future is a living hell because of the androids and cell(android that wants to get his perfect form) .He tells them that the future gohan he knew died of the androids 17,18.dragon ball z wiki