You're young and have an opportunity to learn. Use this time to learn as much as you can about as much as you can. Think of it as an investment in yourself. I was in the same boat once, and was continually told "ya, we'll get you a raise in a couple months, etc...." It never happened, or it was $0.25/hr (like tipping a penny IMO....). But the opportunities to learn new things and work in different departments were always there so I took them, and didn't complain about "doing more work for less money" I looked at it as an investment in myself, that since moving companies a couple times (due to bankruptcies...) has really paid dividends many times over. NEVER turn down an opportunity to learn something, or step outside your comfort zone to learn something. Be as versatile as you can, and eventually it will pay off either with an employer who values and utilizes all of your skills, or with the opportunity of successful self employment.
We've had a couple kids come through here lately and all they want to do is click a couple buttons on a computer and push the big green button on the machine. When I ask if they want to learn how to run the CNC lathe they're response is "that's not what I get paid for, pay me more money and I'll do it". Funny. Nice work ethic....
As for learning G code. Pick up this book
CNC Programming Handbook, Third Edition: Peter Smid: 978 831133474: Amazon.com: Books