Both the GMAT and the GRE include reading comprehension. Passages can be either short (one or two paragraphs) or long (usually up to five or six paragraphs, max). Shorter passages typically are easier because you don’t have as much information to keep track of; for a quick review on what is on these tests, check out these All About the GRE and All About the GMAT guides! Note the subtle differences: on the GRE you’ll be able to skip around between questions, while questions must be answered in order on the GMAT.
So how should your approach change from longer to shorter passages? For longer passages, it makes sense to thoroughly read and take notes on the important information presented (main idea, function of each paragraph, author’s point of view, etc.). Shorter passages, however, will usually only be accompanied by one or two questions; you should read those questions first before looking at the passage to quickly identify the pieces of information you’ll need to find.
For example, let’s say the first question asks about the “main idea” and the second question asks about the “logic behind the author’s use of a specific detail.” You will only have two tasks as you read: find the purpose, and find out why the detail is included. There’s no point in trying to focus on the author’s point of view if it isn’t necessary to answer any of the given questions! Make your job as simple as possible.
Even short RC passages have a main idea. It's often contained in the first few sentences, but beware: the main idea can ultimately appear anywhere in the passage. And remember: creating clear tasks for yourself is an effective strategy for shorter passages, since you don’t have as much text to decipher.