The short answer is YES, you lose points for every question you don’t fill in (up to 15 percentile points per section). This does not mean you should rush through the test to complete them all, but rather complete the number of questions you need to answer based on your pacing plan and guess on the rest. Don’t lose points unnecessarily because you forgot to fill in an answer for every question.
For more insight into the different effects that guessing and leaving questions blank can have on your score click here.
What Does the GMAT Test? How Should I Prepare?