Wikipedia
Business-to-government (B2G) is a derivative of B2B marketing and often referred to as a market definition of "public sector marketing" which encompasses marketing products and services to various government levels - including federal, state and local - through integrated marketing communications techniques such as strategic public relations, branding, marcom, advertising, and web-based communications.
B2G networks provide a platform for businesses to bid on government opportunities which are presented as solicitations in the form of RFPs in a reverse auction fashion. Public sector organizations (PSOs) post tenders in the form of RFPs, RFIs, RFQs, Sources Sought, etc. and suppliers respond to them.
Government agencies typically have pre-negotiated standing contracts vetting the vendors/suppliers and their products and services for set prices. These can be state, local or federal contracts and some may be grandfathered in by other entities (i.e. California's MAS Multiple Award Schedule will recognize the federal government contract holder's prices on a General Services Administration Schedule).
There are multiple social platforms dedicated to this vertical market and they have risen in popularity with the onset of the ARRA/Stimulus Program and increased government funds available to commercial entities for both grants and contracts.