Are gm motor mounts filled with hydraulic fluid?

No, GM motor mounts are not typically filled with hydraulic fluid. Most GM motor mounts use either a hydraulically dampened system (meaning they use hydraulic fluid *within* the mount itself, but not a completely filled chamber) or a combination of rubber and/or polyurethane to isolate engine vibration. There might be some specialized or very old exceptions, but the vast majority do not contain a reservoir of hydraulic fluid.