Gasoline has to vaporize before it can burn. The vapor Pressure determines how fast the gasoline liquid turns into an vapor. Vapors are oxidized by Oxygen (burned) LEO GER.

The space between the liquid and the inside walls of the gas tank could be called Head Space.

Vapor pressure INCREASES with and INCREASE IN TEMPERATURE. Vapor pressure also may increase with a decrease in Air Pressure surrounding the liquid gasoline. But the main source of increasing or decreasing vapor pressure is due to the air temperatures surrounding the liquid gasoline.

Are you talking about those portable gas tanks used for Boats? I have to squeeze a squeeze ball in the gas tank line to pressurise my gas tank. The pressure inside the tank and the suction coming from the fuel pump on the engine pulls gasoline out of the fuel tank through the fuel lines (hose) and then into the engine.

Might check your fuel lines to see if you have any leaks. Sounds like your system was pressurized ok since it squirted out when you opened the cap. My 6 gal metal gas tank will do that too when I open the cap. Or maybe there is a vaccume inside the tank and air is rushing into the tank. But it does make a whoosing noise as air rushes in or out of the tank as the cap it taken off. That's normal for those small 6 gallon portable gas tanks in boats. My truck's gas cap gives me a whooshing sound when I open it up.

How full are you filling your gas tank in your boat? Might want to leave a little head space. Only put the amount that's stated on the tank. You can add some Stabil or Sea foam to the gas to keep it over the winter. Add the stabit to the gas and then run the gas though the motor for about 5 minutes to make sure that the stabil gets inside the carb and engine good. That will keep the gas from gumming up as the volitale compounds evaporate and leave behind the heavy stuff.