I think it's around 8 dbi. Not 100% sure on that but there were some posts here a couple weeks ago where "he that shall not be named" gave us a little theory on antennae design. Those posts have since been removed by the admin.
Instead of me rambling along about antenna GAIN. I thought a few links would be better.
Just be careful with HIGH GAIN antennas, the higher the gain the more directivity the antenna will exhibit both in receive and transmit. You have to insure that the antenna is pointed at your (insert aircraft type here) drone.
I watched a brand new F-16 a guy had spent over a year building fly away (cost over $1100 in 1990's dollars), simply due to letting his transmitter (antenna) point away from the aircraft.
I find nothing wrong with HIGH GAIN Antennas, I use a homemade 7Dbi antenna on my 2.4Ghz side of my ST-16, I just have to watch antenna angle when the H is directly overhead, that's where there is a null or less power being radiated. You can Google for more information than the links I posted.