Will a person be sinful for gifting musical instruments to his friends, being unable to retrieve them, and is it permissible for him to sell the remaining musical instruments he possesses?

If the items are used only for sinful purposes, then they must be destroyed. It is not permissible to transfer them to others, whether by selling or gifting, because that would be cooperation in sin, as Allah Almighty says: {And cooperate in righteousness and piety, but do not cooperate in sin and aggression.} (Quran 5:2). As for musical instruments that you previously gifted, you must retrieve them to destroy them, if possible, along with advising those to whom you gifted them. If the items can be used for both good and evil, then do not transfer them to someone whom you predominantly believe will use them for unlawful purposes.