Time management on the GMAT requires a combination of test taking skills that all play together to improve your timing:
1) Following consistent processes
Like fully processing the prompt information and looking for patterns in the answer choices in quant BEFORE trying to answer the question. Or narrowing the argument down to approx 10 works in critical reasoning again BEFORE answering the question. Those are just examples. The reason why consistent process is critical is because the correct process aligns your approach to the WAY the GMAT tests you. If you answer like you would on a normal test, you'll fall into the reasoning traps the GMAT sets, and then waste huge amounts of time on a single question.
2) Always having an educated guessing strategy.
This one often surprises clients when I share it with them. Here's an example to show why it's important. If, while you're doing your initial analysis of the information in the prompt of a quant question, you find something that rules out 3 of the answer choices right away, then you should immediately note that and cross of those answers. It's because you never know when you're going to suddenly hit a wall on a question. By using the narrowing of choices approach, you set yourself up to guess if you have to within the timeframe of the questions (2-2.5 minutes) and still have a 50/50 shot of getting it right.
During testing simulation, where we coach you live while you're taking a practice exams, we've noticed that lots of test takers have trouble letting go and going back to answer choice elimination when they feel they should be getting the answer. That psychological battle if often what blows timing for a lot of test takers. By eliminating as you go, you setup the guess if it suddenly becomes the only option.
3) Letting go of questions
We spend a lot of time changing test takers mindset about how to get a great GMAT score. Unless you are trying to get to the 99th percentile (which is unnecessary for most programs), you can get a few questions wrong in each section. What's important is which questions you get wrong and in what sequence. The first couple of questions are important to tell the algorithm you're capable of a solid score, but after that, the key is not getting easy/medium questions wrong and then avoiding getting multiple questions wrong in a row. So you can let go of a question (using the smart guessing / elimination approach above) and still get a great score in each section.
Hope this helps.