I haven’t seen very many detailed mass breakdowns of engines that call out the mass of the gimbal system.
For the Apollo Service Propulsion system, the gimbal actuators alone contributed about 20kg to the 300kg engine according to the original spec; that’s a lower bound since it doesn’t include the gimbal mounting & bearing itself, but suggests that gimbals add something on the order of 10%-15% to the mass of the engine.
The Sea Dragon proposal shows the gimbals and actuators contributing 33% of the engine mass, but this is for a pressure-fed engine, so the total engine mass does not include a turbopump -- usually a significant portion of the engine. This engine is an outlier -- the biggest single-chamber rocket engine ever seriously proposed -- so I'm not sure how relevant it is to the question.
Engines in turn contribute typically 1%-2% to the total fueled mass of a rocket stage, so gimbals might be around 0.1%-0.5% of total launch mass.
A 1972 study for modeling of space shuttle proposals assumed gimbals would contribute 0.0007 lbs of weight per lbf of engine thrust; for a 1.4:1 liftoff thrust-to-weight ratio this would work out to about 0.1% of launch mass.
I believe the NK-33 derivatives AJ26-58 and AJ26-62 are nearly identical except for gimbals on the latter, but I haven't found single-source, side-by-side mass figures for them. Aerojet advertises the gimbaled version at 1471 kg versus the base NK-33 at around 1240 kg, which could mean a 231 kg (19%) increase in engine mass for the gimbals. Two such engines boosted the retired Antares 100 launcher, liftoff mass 282000 kg, so if the engine specs are correct, gimbals contribute 0.16% of liftoff mass to that rocket.
