In Russian, "it is getting," and "it gets," is identical conjugation, and the difference in the meaning would be determined by context. However, the English sentence, "It is getting warm in the summer," is an awkward construction. "In the summer," implies a general statement, not a particular one. When in the summer? What part of summer? Why in the summer specifically? "It is getting warm," implies a particular event, such as getting warmer right now! So, a better alternative would be:
It is getting warm because it's summer.
It gets warm in the summer.