How do i multiply gravity with mass in finding potential energy with the equation pe=mgh

That's it !  That's all you need to do.
The formula for gravitational potential energy is

         PE  =  (mass) x (gravity) x (height) .

-- Where it says 'mass', write the mass of the object.
-- Where it says 'gravity', write the acceleration of gravity
       on the planet where you are.  On Earth, it's 9.8 m/s².
-- Where it says 'height', write the height of the object above
       the floor, or above the ground, or the roof ... whatever.

When you put those numbers in and do the multiplication,
the answer you GET is the object's PE.
 
The neatest part is:  If you put in the numbers with SI units ...
mass in kg, gravity in m/s², and height in meters ... then you
don't need any more numbers, like constants or conversion
factors etc.  The SI units all work together, so that the answer to
the multiplication of those 3 numbers IS the PE with units of joules !