The efficiency gap is just a measure of the actual effect of gerrymandering in an election, it doesn't say anything about how to draw a district for future elections.

Compactness tells us nothing about the effects of gerrymandering. Less compact districts correlate with more gerrymandering but that doesn't tell us whether that gerrymandering was effective. Compactness could be a useful tool to limit the effects of gerrymandering when drawing districts.