Have an issue with litigation property and khata. Kindly help

We have a property in bangalore (ancestor property) which had litigation issue. Case is running in KAT (Karnataka appetite tribunal ) and we have given gpa to a person 3 years ago by taking a small amount which he is trying to misuse and now we are trying to cancel the gpa since the KAT court judgement is coming next week in our favor. And the khata is not in our name yet. 1. My query is how much time we have to cancel the gpa. And what are the ways to cancel. 2.will the gpa holder able to sell the property on our behalf. 3.can the gpa holder sell the property without transferring the khata to our name. 4. We have filled a new case to cancel the gpa and sent the notice through ameena which he is not accepting and he says he is always away from bangalore. Can I send the notice through WhatsApp and is it valid in court? Thanks in advance