I had a question regarding gheebah. What if I have committed gheebah but I cannot apologize to the person I did gheebah on. What else shall I do?

You can try asking for forgiveness from them without going into much details. You can also ask for forgiveness from Allah to forgive this sin. If the gheebah has caused the other person serious damage such as financial or family breakdown or something similar, it must be compensated for. Best to be very mindful of such sins and ask Allah to help us in refraining from sins inshAllah