As the gym cables pass over, under and around the gym pulleys, the pulleys roll on their axles and if the gym pulleys are functioning properly, they offer almost no friction to the surface of the gym cables, so any grease would offer no significant benefit to reducing friction. Grease on the cables can come off on the gym user's hands and get transferred to the gym's upholstery and the handles of the gym attachment, making them sloppy and slippery. Also, eventually, some of the grease is likely to be transferred the carpets and walls. Furthermore, a coating of grease on the gym cables can mask defects in the cables that might otherwise be noticed.

If you have vintage gym cables that have no protective vinyl coating, those bare cables do require grease to keep the cables from rusting. However, a better course of action would be to replace those aged cables with the modern vinyl coated gym cables. The repetitive bending of the cables as they rolled over the gym pulleys over the years has probably degraded them anyway, whether the defects are obvious or not. When dealing with gym cables, it is always better to be on the safe side.