GCC states are not military powerhouses, and retaliation would be symbolic and centered around air strikes. GCC states instead project power through hydrocarbon leverage, money politics and foreign mercenaries. The Saudis are by far the largest Gulf military on paper but their Yemen debacle showed how weak and vulnerable their army actually is. GCC participation in the air campaign would definitely give the US/Israel a legitimacy boost though.
A proper analogy as to how GCC states view militaries is as to how they perceive football. It's a status symbol and they want immediate success. So instead of setting up proper infrastructure, esprit de corps and military institutions they buy all this hyper-expensive equipment without much deeper understanding, exactly like how Saudi clubs are currently poaching top tier footballers for exorbitant salaries. Yet the Saudi League hasn't improved much and at the end of the contract the players walk out of 100 million USD richer and nothing has really changed.
As said, a GCC retaliation would mean a legitimacy boost for especially Trump. The US likes to build grand coalitions for its military adventures. Ergo a GCC entry for Trump would mean that he can sell his Team's assessment as correct, even though in terms of actual relevance the Gulfies contribution will be low.
