Gohan had to save Trunks, who had been knocked out by Android 18. When he tried to hide, 17 and 18 blew up the whole area and flew away. When the smoke cleared, Gohan only had one arm.
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Gohan lost his arm by saving trunks from the androids,that was depressing it got blew up by the blast in History of Trunks
gohan lost his arm fighting the androids
I don't know which instance you're talking about but here are the two times that Gohan's arm got noticeably hurt. In the series Gohan is a teenager fighting Cell and his arm just gets broke in the last few little scruffs. In the movie "History of Trunks" the Androids 17 and 18 shot an Adult Gohan with a ki blast that took off his arm(In the alternate future where Trunks came from).
Gohan is 7 when Trunks first appears in DBZ.
Future Gohan cannot transform into super saiyan 2, because the events leading up to (Kid) Gohan's first transformation to said form did not occur in Future Trunks' timeline. Indeed, Cell did not make an appearance in the future timeline because the androids destroyed Earth and its inhabitants.