You want a diesel cap, a gas cap seals fumes in, creates a vacuum. You pull a vacuum, your pump won't pump, and you'll starve your pump for fuel and damage your HPFP too, as the fuel acts as the sole lubricant for the pump. Without fuel, it's oil starved too, and can and will seize.
A gas cap is for gas, a diesel cap is for diesel. Why would you try to even swap them? Always, on a diesel, replace with original equipment. No telling what kind of damage you've done to your fuel pump, and money wasted on fuel filters, by being cheap and buying a gas cap for a diesel fuel tank. Maybe pay attention while refueling, so you don't lose the cap in the first place?
The other issue is if driving in temps below 32F and you have accumulated water in your fuel... it can freeze in the fuel filter, partially clogging it. Get the moisture out of your fuel by running something like Power Service white bottle. Always dose your fuel when it gets below freezing, and always a couple before the first frost of fall, to dry out your fuel system.
