How can I get 31b if my GT score is 88?

You can retake the ASVAB. If you want something, then you want it enough to put in the work for it.

According to goarmy.com, it requires a 91 ST score. It doesn’t say anything about any other line scores.

Try using the link below to see what are all the MOS's you qualify for, all you have to do is plug in your asvab score into the engine and it will show you your results. If 31b does not show up try talking to the Army recruiter to see if you can get an ASVAB wavier for 31b if it's possible. However, I would highly recommend you retaking the asvab having a 88 GT score will honestly limit your career opportunities.

https://m.goarmy.com/learn/understanding-the-asvab.m.html

Get an ASVAB study guide, and retake it when you feel you're ready. Not kidding, books. Reading and paragraph comprehension as well as mathematical ability are what are used to calculate your gt score. So dictionaries and thesauruses are your friends. Play word games, etc. Since youre an equipment repairer, you obviously have the mechanical portion of the GT score. This is how you break down your GT score. Strengthen these areas. The GT score is complied from your Word Knowledge (WK), Paragraph Comprehension (PC) [These two scores are combine to make your VE score], and your Arithmetic Reasoning (AR) scores. These are a measure of general reasoning ability as well as basic communication skills.

Www.march2success.com

This is the tool recruiters use for ASVAB study. Never spend $$ on study guides. I have the FAST class...think it's called BSEP now, also on my Google drive. Msg me if you'd like a copy (all paper / pdf). March 2 success has a great study tool.

You can take the ASVAB 5 total times in your career. This includes your 1 original test at MEPS as an initial recruit. Education centers have GT prediction tools still u believe.

After the holidays if you'd like to shoot me a MSG I can send you a practice ASVAB like the short one you take at the recruiting office to test how your studying is going.

Request a waiver or retake the asvab

You can take the FAST class to help bring up your GT score. But being realistic here, 31B is balanced at the moment. So chances are slim especially if you're currently in a balanced or shortage MOS. The link I included is the MILPER message that defines MOS strength.
http://www.armyreenlistment.com/Messages/MILPER/In_Out_Calls_18_309_20180925.pdf

Retake the ASVAB. There are classes on post that can help you be very successful in retaking it. I went from a 108 to a 132.
Do that and then look into other jobs. MP isn’t as exciting as it seems.

Only thing you can to do is get an ASVAB study guide and retake it. Set your goal realistically, and not exceedingly high. That way you will hit your goal and not be disappointed.you can take it more than once

Schedule a study Class at the Education Center, then apply for a re-test.