Shaking a liquid with gas dissolved in it will raise pressure (you can try this with soda). However, I think your question is not well-posed, as volatile means "1: readily vaporizable at a relatively low temperature" (see http://www.merriam-webster.com/dictionary/volatile), so "volatile gas" is an oxymoron. Almost invariably, a given mass of liquid will require less volume than the same mass of the same substance in gaseous phase, so condensation (the change in phase from gas to liquid) will tend to reduce pressure.