Gate capacitance = Cox * W * L
where,
W is the width of the transistor
L is the length of the transistor
Cox is the capacitance denisty of the gate (in F/m²)
Then u can multiply the final value with the total number of gates ur path see
I have a doubt that whether ,i have to multiply or add the capacitance of the individual gate as the path see, because the substrate of the NMOS is at Ground and Substrate of my PMOS is Vdd.