> I fail to make sense of the claim that there is such an “explanatory gap.” It regards what we would understand if we were to understand something that we currently do not understand. Forgive the muddled question, but: How can we know now what we would understand if we were to understand something we do not currently understand?

Rhetorical nonsense. If I'm a student about to take geometry for the first time, I can certainly have a sense of what I'll understand when I "understand something [I] do not currently understand".

The explanatory gap, IIUC, is rather simple: we can't explain why neurons firing results in us feeling/experiencing the world. This doesn't seem controversial to me.