How do I interpret the requirements to join a specific section of the Marines?

Remember to capitalize the word Marine. It will make your future career much nicer. Contact your recruiter. They can give you the most current info.

GT not a different test, it is a special section from ASVAB. ASVAB contains multiple sub-scores and GT is a compilation from different metrics. You are correct that certain MOS standards require varying scores for GT.

My advice for you is to study for ASVAB re-test. There are multiple videos on YouTube with suggestions and your local library should have a book or two about ASVAB prep with practice tests.

OK, your AFQT score of 30 is where you fall percentage wise in those that take the test and it puts you in Category IV. That will make it very hard for you to get a recruiter to look at you since most will require a Category IIIB or better, so a minimum score of 32. The GT score is a composite score of two of the ASVAB subject areas, in this case Verbal Expression and Arithmetic Reasoning. A quick look says Marine MP requires a GT of 100. With an AFQT of 30, I would be surprised if your GT score is that high. My advice is that if you want this is to look at one of the ASVAB study sites online and study up on the test to bring your scores up to an acceptable level.

Thomas OReilly Now you need more education to pass the military testing that will allow you freedom of choice for training, get your act together and start doing more homework NOW!

All services have MPs, Security Forces, etc. The Marines have a wider field of detail being under the Department of the Navy. Just don’t speak to a Marine about that because they hate their mother!
As far as your scores, you’ll take a series of 10 tests, or what’s know. As a battery of tests which comprise the ASVAB. Your basics, such as math, reading, writing, and then some furthered to speed, mechanical, engineering and such. Ideally, the ASVAB is supposed to generalize where you will best fit within the military branch of service, based-off the end result (scores) and there are a number of those, if I recall, something like 16-18 different scores. One of them is your GT score. That score is you make/or break score for every job! They do have waivers for scores, your recruiter can tell you, I know because when I first joined, I needed one.
All that said, you have plenty of time to practice and get the one score up you need - the magical GT! There are study guides you can buy at Barnes & Noble and online equivalents. Just pull up ASVAB study guide online and a number of them will pop-up I am sure...
You can take the ASVAB again in 6 months and then I believe 1-year from then on out, but don’t quote me on that one!
The biggest challenge you have is no one probably told you any of this, so no you have the opportunity to do yourself better! The higher the scores (GT) specifically, the more jobs that will open up to you!
With all that in mind, I would look at each of the 4 services, and review what my options are with regards to a career as an MP. Do you maybe want to switch over to Investigative services like NCIS or CID. The Army’s CID is pretty unique but it all starts from the initial MP stamp, I’m not sure you can walk right into CID work without some initial street experience behind you (don’t quote me!)
Your best bet is to study-study-study and raise those scores! You have plenty of time, so don’t let a recruiter rush you, EVER, with your decisions - YOU make the calls, and do it right up front! Find Veterans on here whom have or do the work you are thinking of getting into ask ask them questions. Don’t just take the word from your recruiter - there are some very good ones and some pretty poor ones out there like any line of work! Talk with family and friends about the military before you make the jump. You might also consider the reserves and go join the local PD, State Police, as some have possible waivers for your time on duty. Why not build two retirements at once?

Just a few things to get you to think about your future!

Just study as much as you can between now and when you retake it. Just do your best and study for it like your dreams depend on it (because they do if becoming a Marine MP is your dream.) Good luck, buddy.

During your enlistment process your recruiter schedules these tests for you. Usually done at a central location.

Here is the wikipedia article which explains what the various terms on the ASVAB mean.
https://en.wikipedia.org/wiki/Armed_Services_Vocational_Aptitude_Battery
Your total score (AFQT) is what it sounds like you got a 30 on, whereas the Marine Corps requires a 32.

The GT (General Technical) is a composite of the VE (Verbal Expression) & AR (Arithmetic Reasoning)
VE (above) is a composite of WK (Word Knowledge) & PC (Paragraph Comprehension)

Basically, these are the three sections to focus on to increase your GT score; Arithmetic Reasoning, Word Knowledge, & Paragraph Comprehension. The same items you would study for the SAT or ACT tests. A lot of this will get better as you go through High School, but purposeful practice is essential for massive gains.

Thomas OReilly You are a junior, so you have time to study, and study hard. Utilize one of the study guides to bring your scores up, and take the practice tests to measure your progress. Make sure you keep up with your High School classes and your physical fitness. Keep us posted on your path to becoming a Marine.

https://asvabpracticetestonline.com/

Study hard man... go to your local library and take out their most recent asvab study guide... focus on studying and getting in good shape. Best of luck in the future.

There are ASVAB study books and classes you can take. You will need at the min an AFGT of 32 to pass the ASVAB. Your line scores will dictate what mos's you qualify for.