What is the grammatical function of the word "Go" in this sentence spoken by astronauts?

I read it as a predicate adjective meaning roughly that "you" (the astronaut team in Apollo 8) is ready for launch. This usage is also seen in the idiom "all systems are go." Here's how the Oxford English Dictionary describes the usage:

3. colloquial (originally U.S.). Chiefly in predicative use. Functioning properly; ready and prepared; fulfilling all conditions necessary to proceed. Originally with reference to devices and systems in a spacecraft.

The usage goes a little further back than NASA. Here is an excerpt from a 1958 book Rocket to the Moon, which is a researched account of what a space program capable of going to the moon would look like:

"Azusa-go! DOVAP-go! Telemetry one, two, three (etc.)-go!"

Trouble anywhere and the report is "No go!" and down goes the hold switch. All come in "Go!"

[...] "T minus 30 seconds. All systems-Go!"