Gerunds are typically used to talk about an instance of an activity (as a noun) that is actually happening now, has happened, or that the speaker/writer has a high confidence will happen. Infinitives are used to talk about a possible-but-might-not-happen or desired instance of an activity.,
Here's how that would evaluate with your examples:
Do you agree postponing this notification for 2 days?
You are in the process of receiving a notification. Notifications are typically short and instant so this sounds weird. "Do you agree postponing this 10GByte download for 2 days" makes more sense, for example.
Do you agree to postpone this notification for 2 days?
You haven't seen the notification yet. You don't know what the notification is.
Unless your notification is large where receiving it is a long process - which is unlikely as then it would not be a mere notification - the second example is the one that most likely makes sense.