hi stanfordstud,
You cannot assume that a 700+ SAT score on each section will necessarily yield you with a 700+ GMAT overall score.
The GMAT Verbal is actually more like SAT writing than it is of SAT Verbal. No fancy vocabulary is involved. If you are good with reasoning, grammar, sentence structure, and of course---reading comprehension than you might do alright.
The GMAT Quant also involves a bit more reasoning than pure calculation--so the style of questions is a bit different.
(For more info on GMAT vs SAT:
https://www.gmatpill.com/about-the-gmat/gmat-vs-sat/ )
Only you know your learning style best. If you have 3-6 months of free time (you are not working, for example) and you are tight on a budget, then by all means self study as long as the material you are using is helpful for you.
But if you are interested in saving time (you have a job and limited time for yourself) and getting results faster---then we can help. Take a look at some free videos on our site and see if our thought process is the right learning style for you.
Sure you can spend countless Saturdays digging yourself through a book. But I'm sure you value your social time and time away from work. If you want to reduce the amount of time for you to reach your prepared state to take the GMAT--don't be afraid to look for help.
My advice: take a practice exam from mba.com and see how you do. Then assess how much time you can budget and if it's worth it for you to seek additional help.
Hope that helps!