Fracturing systems with frac valves having gates for controlling flow of fracturing fluid are provided. In one embodiment, a fracturing apparatus includes a frac valve having a housing with a cavity transverse to a bore. A gate is positioned in the cavity and is movable between open and closed positions to selectively block flow through the bore. The gate includes a seal assembly positioned to seal against a sidewall of the cavity to surround the bore and block flow through the frac valve when the gate is in the closed position. The seal assembly can be actuated between a less-energized state and a more-energized state following movement of the gate within the cavity from the open position to the closed position. Additional systems, devices, and methods for fracturing are also disclosed.