A torque converter is a hydraulic pump and hydraulic motor in the same housing. In this case, they use turbine L.O. as the hydraulic fluid. The hydraulic pump is driven by the starting means (electric motor; diesel engine; expander turbine).
The reason for using a torque converter is because the starting means can't be directly coupled to the turbine-generator shaft to break the shaft away from zero speed to accelerate it to purge speed and then to self-sustaining speed (the speed at which the turbine can derive sufficient power from the hot gases passing through the turbine to keep itself spinning without any assist from the starting means through the torque converter).
Essentially, the torque converter is a "clutch" with slip, often coupled to the turbine-generator shaft through a jaw clutch or SSS (Shifting, Self-Synchronizing) clutch--to provide a way to allow the starting means to run at rated speed to develop and apply torque to the turbine-generator shaft as it's accelerating from zero speed to self-sustaining speed. (If you've ever ridden in or driven an automobile with an automatic transmission, it's essentially a slipping clutch that is a hydraulic pump and hydraulic motor for all intents and purposes.)
The output ratio of the torque converters used on GE-design heavy duty gas turbines is usually listed on the torque converter manufacturer's nameplate, and it has varied slightly over the years depending on the vintage of the machine, and the type of starting means (electric motor; diesel engine; expander turbine). I think I've seen ratios from 1.8 to 2.6, if I recall correctly.
Hope this helps!