Combustion appears only in gaseous phase. Liquids need to vaporize in order to burn. Considering temperatures in combustion chamber this happens fast, but still part of the energy is wasted in this process. This energy varies from liquid to liquid and is called latent heat of vaporization or enthalpy of vaporization (∆Hvap). It's expressed in KJ/mol and is pressure dependent - therefore not constant even for same liquid. Also, more energy is needed as pressure rises.
As cryogenic liquids are stored at very low temperatures, they are first heated till the critical temperature, then vaporized, and finally heated to flash point when they ignite. Each step is a waste of energy compared to gases at standard temperature. If gasses are pressurized to high extent than they bring additional energy to rocket. Overall difference is not significant, between 3-5% of specific impulse, but neither negligible.
In order to precisely calculate this waste you must know working conditions of the engine such as: O/F ratio, combustion pressure, heats of vaporization at given pressure, ambient pressure etc. For reference hydrolox engine working at 68,05 atm has 4,7% lower specific impulse if you compare LH2-LOX and GH2-GOX. Therefore best in terms of specific impulse are gas-gas, followed by gas-liquid and the worst are liquid - liquid engines for the same oxidizer/fuel mixtures.
The reason we still prefer liquids is because they are denser and require less materials for rocket storage. Gasses should be compressed to several hundreds of atm in order to achieve reasonable densities. To store them you need thick walls which means a lot of dry mass added to the rocket.
At the end, despite poorer performance, liquids appear victorious. As far of combustion instabilities, liquids change volume significantly up to hundreds or even thousand times when vaporized and liquid - liquid engines are more prone to instabilities than the other two types. Instabilities rise with the increase of combustion chamber volume. For very big liquid fueled engines, like Rocketdyne F1, its a real feat of engineering to make them disappear.