Control over English skills - The most important thing you can do to increase your English level while studying for the GMAT is to be an active learner and take control of your English skills.
Certain level of knowledge - You need to have a certain level of knowledge of the English language to pass the GMAT with a high score.
Practice is the key - Practice the English language as much as possible. Listen to English podcasts and watch English-speaking shows, movies and documentaries.
Stick with English newspapers - Read in English in an active manner. Stick with newspapers and magazines that cover current events.
Use a dictionary - While preparing for the GMAT, use a dictionary and make a list of new words. Aim to use some of these new words.
Try to understand words from context - words do not exist in a vacuum, but their meaning in a sentence is part of the context. Therefore, it is important to understand what is around the word in order to make sense of it.
Similarities to other English languages - Note that other languages have similarities to English (such as Latin-based languages). If you know one of these languages, you can use it to your advantage.
Sign up for ESL - Sign up for ESL (English as a Second Language) classes, which may prove useful along with using the many free resources available for ESL students.