Is the pressure of a gas proportional with the concentration of a gas? Is the pressure of a gas proportional with the concentration of a gas?

Because you can also use the pressure instead of the concentration in the  equilibrium condition.
Assuming your gas follows the ideal gas law, you have $PV=nRT$. Setting $c=\frac nV$ yields $P(c,T)=cRT$.

If the temperature is fixed, then $P(c)=(RT)c$ indeed tells us that the pressure of a gas is proportional to its concentration.