How much electricity does a 1 GW plant actually produce if the plant's efficiency is 20%?
A power plant rated at 1GW can produce 1GW of power, at the rated conditions.

If it has an efficiency of 20%, then it will be consuming 5GW of energy in some form to do that.

If the power plant is (say) thermal steam, then the calculations are fairly easy, because we can assume that it can do this continuously, as long as fuel arrives. It will generate 1GWh of energy in 1 hour. Note that 20% efficiency is pretty poor, archaic even, for a combustion based plant, but might be reasonable for geothermal (low temperature) sources.

If the power plant is solar, then 

(A) it's weather dependent (don't work well in cloudy weather) 

(B) time of day dependent (don't work as well when the sun is low in the sky) and 

(C) the efficiency is less relevant than for fuel burning plant, as it gets whatever sun it does for free all the time (subject to a and b above), though cost of plant, installation and real estate to deploy it will increase with lower intercept efficiency, so that will continue to be improved by manufacturers

We come back to the rating conditions. Is that 1GW at solar max, best time of day? Or is it average between the hours of (say) 10am and 4pm? For what the 1GW means, you will need to read the fine print for what it really means. 

Once you know what it means, as a function of time of day, and as a function of seasonal weather, then you can integrate the amount of energy that the solar plant will produce over a day, or a year.