... Most of the modules that you speak of do not discuss polarization, because they usually have "diversity" antennas, or two antennas arranged orthogonal to each other. The receivers (the chips contain at least two separate receivers) continually sample each antenna to see which one has the higher received power and switches to that one. This makes reception possible under most conditions.....
As for summing both the H and V gains, I've never seen that - usually the gain of the intended use is published. For instance, an antenna with a V gain of 2.15dBi and an H gain of -30dBi would still have an approximate overall gain of 2.15dBi.