Not necessarily stronger - you get 10mw and 100mw for wifi
and probably the same for GSM

The difference is that wifi is 2400mhz mostly Line of Sight
(think of signals reflected off of walls)
whereas
GSM has a band down at 900mhz which means the signal can
penetrate many more things. like leaves on trees, walls, etc.

I have stood in open areas and used my n900 wifi from
2km to one of our ubiquiti towers (bullet2hp with a 15db omni),
but I needed the 100mW setting to get a reliable connection

Compare that to getting a BTS signal at nearly 50km distance

GSM 900 is orders of magnitude better for getting long-range
communications.
It is looking like it is also orders of magnitude more difficult to hack.

Given the powerful potential in the world of having GSM phones
which could simply talk to each other without paying tribute
to the Telco overlords, it is easy to see why handphone
manufacturers are bribed and threatened away from putting
this simple feature in their handsets.

But hey, if it can be done at all, it could only be done on the n900.
It would be impossible to do it on any kind of Android machine, ever.