There is no direct correlation of rank between a civilian GS employee and a military officer. However, the responsibility of a GS-13 is similar to that of a Major (O-4). A Major can be a supervisor for a GS-13, but a GS-13 usually will not supervise a Major. In some organizations a GS-13 may have the same level of responsibility as a Lieutenant Colonel (O-5), but it is not that common. Like anything else in the military, there are exceptions to almost every rule.
Special agents are paid as federal law enforcement officers, and subject to locality and LEAP (Law Enforcement Availability Pay) - LEAP is essentially required overtime in the amount of 25%. If the junior SA (GS7) makes $50k and operates in an area with a 25% locality pay, his pay will be 50+12.5 = 62.5k. Add LEAP and you get 62.5 + 15.63 = $78,130 before taxes and deductions. A GS13 will be in the six figure territory. Additionally, federal law enforcement are entitled to retire after 20 years of service (like the military) instead of the 30 typically required for other federal positions.