GMRS Radio and Antenna advice

Hi all.
I am new to this site and fairly new to the two-way radio arena. I have a few questions. First, the scenario: We are interested in supplementing our cell phones with GMRS radio communications in southern Colorado, land of spotty cell phone coverage, where we have our second home. The head of our local volunteer fire department has a 50 watt GMRS base setup with a mast mounted antenna which he monitors 24-7 for the sake of locals who may need to reach someone in case of an emergency. I know that this is outside of the intended application of GMRS, but it is what it is. He is about two miles distant as the crow flies with a couple of mountain peaks in between and our communications (his base and my 5 watt Midland) are exceptional. I know that the terrain (mountain with lots and lots of long-needle pines) is not condusive to really good range but so far, the results of our tests have been impressive. I know that VHF should be best in these conditions, but the cost of a business band license is pretty steep and I do not have an amateur license (my lady does), so we are going with GMRS for which I do have a license. I am thinking of a 45 watt base (ICOM UHF or similar, w/power supply) with a mast mounted antenna and am curious about antenna gain. I have read that anything over 3db gain is not optimum in UHF. Can someone shed some wisdom on this? Also, for auto (mobile) units I am looking at 4 watt ICOM or Black Box portable UHF units with rooftop antennas and hand mikes. Does the antenna performance benefit from being in the center of the roof or is it just as beneficial off to the side or even clamped to a luggage rack bar? Info I have indicates height is the issue, not necessarily location in the center of a horizontal plane like the roof. The previous antenna gain question pertains to the mobile or auto units also.
Thats probably enough question for now. Any help is truly appreciated.