Is inner joy over what afflicts an oppressor considered gloating?

It is commendable for a Muslim to pardon, forgive, and refrain from gloating. Allah admonished Abu Bakr Al-Siddiq when he intended to stop providing for Mistah after the incident of the slander (Al-Ifk), whereupon Allah revealed: "And let not those among you who are blessed with grace and wealth swear not to give [aid] to their relatives and the needy and the emigrants for the cause of Allah, and let them pardon and overlook. Do you not like that Allah should forgive you? And Allah is Forgiving and Merciful." [An-Nur: 22].

There is no sin in supplicating against an oppressor. The Prophet, peace and blessings be upon him, supplicated against the tribes that betrayed his companions at Bi’r Ma’unah, and Moses, peace be upon him, supplicated against Pharaoh and his people. It has been reported in a hadith that the supplication of the oppressed is answered, and that Allah will grant them victory, even if after some time.

Based on the foregoing, there is no blame upon you for the joy you feel in your heart.