Hi tm110.
The best by far way to prepare for the GMAT is to use topic-by-topic prep. What I'm getting at is that, because the concepts that are involved in GMAT questions can be fairly well categorized by topic, you can work on developing your GMAT related skills one topic at a time. In the case of each topic, you would learn all about the topic, learn how to answer questions involving that topic, and then practice by answering dozens of questions involving that topic.
I think that, if you carefully learn about GMAT related topics in that way, and if you develop understanding of the logical underpinnings of topics, rather than simply memorize information, you will either score high on the GMAT over the summer, once you get a chance to take it, or retain a lot of what you learn, so that, once you are ready to prep and take the GMAT, you will have given yourself a head start by beginning to prepare now.
By the way, in case you are not aware, while the details are not yet available, GMAC has said that, because of the closures of test centers, the plan is to make available a way that people can take the GMAT at home. Whether that GMAT will be considered the same as the test center taken GMAT is not clear yet, but it could turn out that the take it at home option is a viable one for you.
For some more tips on how to prepare for the GMAT, you could read this post on
How to Score a 700+ on the GMAT — A Mini Guide for Success.