Would I lose the reps which I gained by posting questions in proposals which get closed someday?
Yes and no.
Technically speaking, the proposal being deleted would mean you no longer get the reputation for anything that was on the proposal. It would all be removed the next time your reputation was recalculated, if ever. There are no special rules (e.g. 60 days, 2 upvotes) for keeping reputation permanently on Area 51.
However, Area 51 does not perform automatic reputation recalculations on deletions. So barring a manual recalculation at some point, the reputation would never get subtracted out for the deletion.