On the highway between Bogotá (6 hr.) and Bucaramanga (2 1/2 hr.), frequent buses reach San Gil from most major cities on the Caribbean coast and central Andes. The Terminal Principal is located 3km west of town (a COP$4,000 taxi ride). Buses for Parque Nacional Chicamocha depart every 20 minutes or so during the day and take about an hour.
For tourist information, your best bet is with one of the agencies, though keep in mind they’ll likely be trying to sell you something. There are several ATMs in town, as well as a tourist police station at Carrera 11 at Calle 7.