in gsm900 there will not be any problem. Just a higher amount of pbgt handovers in the overlap zone, but nothing dramatic.
you can find your antenna patterns on the antenna vendor website, or on the vendor catalogue (www.kathrein.com , RFS , alan dick ,…)
and the antenna gain is measured in dBi, and range usually from 5dBi to 22dBi. When you’re talking about 43dBm, I guess it is the output power from the BTS.