Power gain is like playing with blocks. Imagine you have a block that can produce some power. This block has a number written on it, let's say 10. This number 10 means that the block can produce 10 units of power.
Now imagine that you have another block that can also produce some power. This block has a number written on it too, let's say 5. This number 5 means that the block can produce 5 units of power.
If you put these two blocks together, their power combines. So the total power of the two blocks combined will be 10+5=15.
This means that the power gain is 15, which is bigger than each of the blocks individually.
So power gain is when you combine the power of multiple blocks or sources, and the total power you get is greater than the sum of each block's power. It's like adding up the power of all the blocks, and getting a bonus amount of power!