I'm trying to translate the phrase:
Sicher regnet es gleich...
What is the meaning of "es gleich" in that context?
I don't think "equally" fits so well
The expression is not es gleich. The es belongs to es regnet (it is raining, it rains). In this context, gleich means in a moment, immediately or in a little while.
The es comes after regnet, because the sentence is a simple positive sentence where, in German, the verb takes the second position (Verbzweitstellung) after the sicher (surely).
So the sentence means something like