Air carrier Captain reported while entering gate area to park; a catering truck was parked near the gate. The FO shouted 'stop' to avoid a near collision.

Narrative

Parking [Gate] XX at ZZZ. Service vehicle (probably catering but not fuel) was parked next to aircraft on adjacent XY gate; closer to XY but not obviously clear of [Gate] XX. I stopped aircraft on XX lead-in line one aircraft length from gate and awaited wing walker assistance (Docking Guidance System was operational and working). Wing walkers came out and as he approached on the XY side gave a clear signal to proceed. Vehicle was still present. As I continued toward the gate Fist Officer (FO) suddenly yelled stop! stop! stop!" Guide man suddenly crossed hands to indicate stop. I stopped the aircraft abruptly. The service vehicle driver had by now realized his position was not an appropriate place to be parked and decided to back out. We avoided contact but not sure by how much. Vehicle was gone by the time we parked and finished the check list.Cause: Numerous factors contributed to this near collision. First; there is a growing problem with service vehicles (usually catering trucks) parking to take a break at inappropriate locations - usually in the middle of an unoccupied gate. In the past six months I have stopped at the top of a gate and waited for the driver to finish a phone call; finish texting and even awaken from a nap. All while parked in the middle of an active gate. This incident was typical. These drivers need to be directed to a safe location when there is no immediate work to perform - not an unoccupied gate. The second contributing factor is the protective area paint; or lack thereof between XX and XY at ZZZ. The standard safety zone white with red paint is present on the left side of XX. Simple; helpful and easy to interpret if there is a vehicle or object within that zone. On the right side of XX however the safety zone paint is absent and is replaced by a red "Fire Lane". Great for fire trucks and emergency vehicles but of no help to parking an airplane. As pilots we have become accustomed to referencing the red/white safety zone paint to safely navigate in the often crowded ramp space. When that paint is missing and we have obstacles within close proximity; it takes away a layer of safety. This was a perfect storm compounded by the missing safety zone paint. Suggestions: Educate these airport vehicle drivers that parking next to or on a gate is not appropriate - especially while taking a break. Also maybe re-prioritize the Fire Lane paint ."