Legal and social
Make sure to check with the telecommunications regulatory agency in your country to understand if it is possible to operate a GSM network legally, and how. If there doesn't seem to be a way, inform yourself as to how much trouble you'll get in if you operate a non-legal network.
In almost every country, GSM frequencies are tightly controlled by national regulatory agencies and leased via multi-year or multi-decade concessions to commercial carriers. In some cases, the entire usable spectrum for GSM has been allotted to existing providers. In other cases, there might be some spectrum left over to use. It is important to note that "squatting" on spectrum that has been assigned to a carrier likely implies getting into more trouble than using un-assigned spectrum without permission, although both likely carry stiff penalties.
In countries that use 850 mHz for GSM, you could consider using 900 mHz instead, part of which falls into the ISM band, a harmonized unlicensed portion of spectrum recognized in most parts of the world.
What Rhizomatica has done is to work with the national regulator in Mexico (many other groups and organizations were part of this process), IFETEL, to create social concessions that allow communities and civic groups to operate their own networks. This effort is bolstered by more specific rulings that allot un-assigned GSM spectrum for rural and community use.
What we have been able to achieve has been based on lobbying and advocacy at the federal level, but always accompanied by facts-on-the-ground (as in we had at least one network already up and running before asking for permission) and social organizing. From the beginning, we involved rural and indigenous communities, many of which are municipal governments in their own right, in the advocacy efforts. This helped to lend political and moral weight to our demands for spectrum access. For as Frederick Douglass wisely said: "Power concedes nothing without a demand. It never did and it never will."