No matter how you slice it, an electric pump will be expensive to operate.

When you run your 4" gas pump, how long does it take to "top off" the pond? Also, how many inches will it rraise for the same time frame?

A 2", 2HP, High-Volume, Hi-Capacity, swimming pool pump can handle about 160GPM at 50' head. That is what my "Century Brand" pump is rated---250GPM at open flow--It has a pressure max of 75 psi. Couple that with a Sylvania industrial timer and you'll be good to leave.

For energy consumption--When I ran my pump 2 years ago,12 hours per day, it cost $105 a month to operate.