First off.. I know nothing about electrical systems.
That's the first step: systematically learning about marine electrical systems, using your boat's current setup and likely use as a model to orient you to all this new information.
As part of this first step, draw a schematic of your boat's wiring, and draw up an inventory of everything: wire sizes, appliance make and model, fuses, etc.
Before re-wiring, learn how to select marine grade wire, pick the right wire sizes, make proper connections, and run wire properly on a boat. Check out the AYBC standards.
Suggestion: Nigel Calder's Boatowner's Mechanical and Electrical Manual and Wing's Boatowner's Illustrated Electrical Handbook. And take a marine electronics or marine maintenance course from eg USPS or CPS.
What gauges should I buy and how would they work?
If your system is as simple as you describe, you might start with a 12 volt voltmeter. A fully charged battery is about 12.6 volts after any charger and appliance attached to it has been turned off long enough for it to settle to its resting charge state. With a charger of some kind attached and operating, the voltage will be a little higher than that. Resting voltages lower than 12.6 volts generally indicated some state of discharge, if the measurement is accurate (good gauge, good wire connections, etc).
12.1 50% (Warning!)
10.5 0% (dead battery)
Note: these are approximate numbers. Actual charge states and significance vary with type of battery, temperature and other factors.