Do NOT use apps to study for the GMAT. They will not suffice. If you're spending 10-15 minutes on a GMAT app while waiting for the bus, in between classes, etc. then that's great, but your actual serious study time needs to be spent with official material and high-quality practice tests, using books and your computer.
These are the mandatory books for any serious GMAT test-taker, in my opinion:
"¢ The Official Guide for GMAT Review (2015 edition):
https://amzn.to/1HCICKb
"¢ The Official Guide for GMAT Review (2016 edition):
https://amzn.to/1Gox0qq
Since approx. 25% of the questions in the GMAT OG16 are NEW. I recommend you study with the 2013 or 2015 edition FIRST (they are identical), then purchase the 2016 edition and do the new questions and REDO the old questions as a "test" to make sure you really understood them the 1st time around.
"¢ Manhattan GMAT Fractions, Decimals, & Percents:
https://amzn.to/1JIl2JB
"¢ Manhattan GMAT Algebra:
https://amzn.to/1HBGKzr
"¢ Manhattan GMAT Word Problems:
https://amzn.to/1KBzkNz
"¢ Manhattan GMAT Geometry:
https://amzn.to/1GUHaNJ
"¢ Manhattan GMAT Number Properties:
https://amzn.to/1KBA1Xd
"¢ Manhattan GMAT Sentence Correction Guide:
https://amzn.to/1KBA6KD
"¢ PowerScore Critical Reasoning Bible:
https://amzn.to/1HCJe2p
You don't really need an RC book as there are plenty of passages online to practice, but you WILL need to develop a solid strategy to reading the passages and dealing with the answer choices.
Also, I didn't include the Verbal and Quant Review books from GMAC. Most test-takers don't even make it all the way through the Official Guide, so I wouldn't bother getting them until you really need them. These books will keep you busy for awhile.
Best,
Vivian