Is it permissible to accept a gift from a contractor whom my previous company dealt with because of me, knowing that I have left the company and am no longer an employee there?

Since the questioner has left the company for some time, the amount is not considered gifts for workers, bribery, or commission. Rather, it falls under acts of kindness, a reward for a good deed, or a gift in return for a favor. This is a good matter with no objection, as Allah says: ﴿Is the reward for good [anything] but good?﴾ The gift is a prophetic tradition that increases love among people, and the Prophet, peace and blessings be upon him, encouraged it by saying: "Give gifts to one another, and you will love one another."