Is it obligatory, recommended, or forbidden for me to ask for forgiveness again from someone who previously forgave me, after my intention changed from seeking absolution for backbiting to seeking to gloat?

Scholars have differed regarding the obligation of seeking forgiveness (tahallul) for backbiting. Some do not consider it necessary and deem seeking Allah's forgiveness (istighfar) sufficient. Since you have sought forgiveness from your companion, your conscience is clear, and you are rewarded for your action as long as your intention was to seek forgiveness. Nothing is binding upon you concerning what occurred to your intention thereafter. However, strive against your self so that you do not become involved in gloating, and do not inform him of anything due to the harm that would result from informing him.