[shubham]

what would the equilibrium expression be if a reaction equation had both aqueous and gaseous parts. Would I convert the gaseous pressure to chemical concentration equivalents using RT?

Or would I just use pressure in atm along with molarity?

[Borek]

Or would I just use pressure in atm along with molarity?

I think the correct unit to use is bar, but yes, you are combining pressures with concentrations (technically they are all converted to activities, so there is no unit mixing)

[Meter]

While schools fail to teach this, each term in an reaction quotient is divided by a standard activity. That is, all molar concentrations are divided by 1 mol/L and all pressures are divided by 1*10⁵ Pa (which is exactly 1 bar), so units cancel out. This is significant too, as dividing by 1*10⁵ changes the numerical value of your pressure terms significantly.