Macroeconomists tend to study GDP in terms of macro variables.
Microeconomists study general equilibrium (among other thinga of course).
Is there theory about the relation between general equilibrium and GDP? Where should I look for this?
Macroeconomics nowadays tend to require microfoundations. As a result, general equilibrium are more of a topic for macro than micro analysis. The dynamic stochastic general equilibrium (DSGE) model and its variants underlie much of macroeconomic analysis nowadays.
You would probably be interested in the identification of Balanced Growth Path (BGP) . It one of the topics covered in advanced macroeconomics.
David Romer gives it a good treatment in his book advanced macroeconomics.