* He probably goes to a barber, not a salon. Gibbs is portrayed as a traditional, no-fuss man. He likely wouldn't frequent a salon with lots of stylists and pampering services. A classic barbershop aligns much better with his personality.
* He probably has a regular barber. Many people who maintain a consistent short haircut find a barber they trust and stick with them. Given Gibbs' adherence to routine and his preference for tradition, it's likely he has a regular barber.
* He likely asks for something simple. Gibbs' haircut is a very straightforward, classic style. He wouldn't be asking for anything elaborate or trendy. He'd likely tell the barber to just keep it short and neat.
In conclusion, while it's never shown on screen, the most likely scenario is that Gibbs goes to a traditional barbershop and gets a simple, short haircut from a regular barber.