Typically the job of protecting a high-ranking congressional member like Gowdy would fall to Capitol Police while he's in Washington and local or state police when he travels to South Carolina.
The Capitol Police made a plea this morning to both the Secret Service and the Marshals when they realized there was a conspiracy to kidnap Gowdy's family and assassinate the congressmen when he showed up with the ransom. The plan, according to FBI analysts called to help, had little to do with money and more to do with silencing a major investigative force in congress.
The Secret Service had to decline, being stretched so thin with President Trump's large family, but the Marshals were happy to oblige. Within the hour, Gowdy's family was safely escorted to an undisclosed location and a working apartment was set up inside the Capitol building for Gowdy so he can continue to do his job. He will also have 24-hour armed protection from federal agents, both uniformed and plain-clothed.