According to Feynman's Lecture on Computation (Problem 5.1, p. 148 ) you can extract $E=kTN\log 2$ out of two copies of a random $N$ bit random tape. From this we can conclude that it takes the same ammount of energy to create a copy.
Now assume a fictional two dimensional universe that follows the rule of Conway's game of life. The gun then creates copy after copy:
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Where does the energy come from? Is it possible to show that the initial conditions plus the set of rules create an infinite ammount of gliders?
Your gliders are disappearing at the bottom-right corner, for the law of conservation of mass and the first principle of termodynamics their mass and energy have to be changed in something else.
As long in your universe you have no entropy (it cannot arise from the rules of Life), you will be able to create as much gliders as you want.