GSA also has “guards”. THey got past the guards. No notice in any of this who is guarding this particular building.
I've been in the Hale Boggs Building before - I think the entrance/exit security is handled by a GSA contractor. Definitely not U.S. Marshal's Service personnel, like at the federal courthouse right across the street.
Still, it's a standard setup - metal detector, present your photo I.D., get a Visitor Pass. There may be additional layers of security on the floor where the congresscritters' office space is located - maybe the same contractor has that detail, maybe not.