Is it permissible to give gifts to a troublesome neighbor to avoid his harm, and not out of love for him, and would that be considered bribery?

That is not a bribe; rather, it is a courtesy, good character, and permissible managing of affairs, based on the saying of the Messenger of Allah, peace and blessings be upon him: "Defend your honor with your wealth," meaning: you give to the poet and to those whose tongues you fear.