When I have to work with spaghetti code, the first thing I work on is modularization. Find places where you can draw lines and extract (more or less) independent pieces of the codebase. They probably won't be very small, due to a high degree of interconnectedness and coupling, but some module lines will emerge if you look for them.

Once you have modules, then you're not faced with the daunting task of cleaning up an entire messy program anymore. Now, instead, you have several smaller independent messy modules to clean up. Now pick a module and repeat on a smaller scale. Find places where you can extract big functions into smaller functions or even classes (if G2 supports them).

This is all a lot easier if the language has a sufficiently strong type system, because you can get the compiler to do a lot of the heavy lifting for you. You make a change somewhere that will (intentionally) break compatibility, then try to compile. The compile errors will lead you directly to the places that need to be changed, and when you stop getting them, you've found everything. Then run the program and test everything! Continuous testing is crucially important when refactoring.