how much compression / hp gain?

Assuming:

bore: 3.81
stroke: 3.39
stock CR: 9.0
same head gasket thickness as before

Then:
V stroke = (bore/2)^2*Pi*stroke = 38.65 in^3
V compression = V stroke/8 (derived from CR=(vstroke+Vcompression)/Vcompression) = 4.83 in^3

20/1000 skim = (bore/2)^2*Pi*0.02 = 0.23 in^3

So your new V compression = 4.83-0.23 = 4.6 in^3

New CR = (vstroke+Vcompression)/Vcompression = (38.65+4.6)/4.6 = 9.4:1


But then, according to this site,

http://www.bgsoflex.com/crchange.html

It should give you only about a 1% increase in HP

However, depending on your reason for having the heads skimmed in the first place,
You may still notice a difference if your previous setup had some kind of power loss due to minor leaks.