su ei Petros, kai epi tauth th petra oikodomesw mou thn ekklhsian
The Greek has "epi taute te petra", and taute typically means "the same". So a literal translation could well be: "You are Petros, and upon this same Petra I will build my Church." The gender of the noun here doesn't matter one fig, because we are *explicitly* told in John 1:42 that Peter is a Greek translation from the Aramaic Cephas.
You're right..God of course is the Rock elsewhere in Scripture. But you can't take one metaphor from a completely different place in the Bible and read it overtop of a second metaphor so as to distort all meaning of the latter.
So yes, God is a Rock. And Peter is a Rock, as this passage plainly states. Our challenge is not to mix the metaphors but to understand how the one relates to the other.
.Pe 2:5 Ye also, as lively stones, are built up a spiritual house, an holy priesthood, to offer up spiritual sacrifices, acceptable to God by Jesus Christ.