Sleep Apnea has a few different ratings. 50% is the most common, as that rating requires a breathing assistance device for treatment. Most with OSA use a CPAP, hence 50% rating. You may have OSA of a level that is not symptomatic, or that makes you tired, but not rise to the need of a CPAP, or you may have SA or a sort that is too severe for CPAP, so there is no guarantee of 50%.
Sleep Apnea is not a presumptive condition, so the gulf war service doesn't help. There needs to be a nexus between the OSA and active duty service. The easiest way to do this is for the diagnosis to happen while on active duty (training counts), but there are other ways to link the two. The most common way is for it to be secondary to a service connected issue. I.e. a doc writes up that your OSA is as likely as not (50%) chance that your broken back (service connected) caused weight gain due to inactivity, which caused the OSA. Some have tried to link the symptoms starting while on active duty, i.e. your physical exam showing that your sleeping problems started on X date, but the diagnosis came later (would help if the exam was done just before or after an activated period).
The difficult problem for reservists is if the injury or illness started while deployed or in training or otherwise activated (I believe 2 weeks in the summer qualifies), it is compensated, but the time between is not covered. If the condition started while at your non-military job, no VA compensation. This is more obvious is trauma cases as you can pin down the event that started the broken arm or what have you. OSA it can be hard (impossible?) to pin down the exact date of the start of the condition when you need to argue it started at a time besides diagnosis. Reasonable disagreements between the two sides, the tie is supposed to go to the veteran, however, the date of diagnosis is pushing the judgement away from service connection and may be difficult to overcome.
As you can tell from that article, there is some disagreement with the rating level for OSA and a great deal of suspicion on why so many claims are coming in. Nobody seems to note that few service member now being newly diagnosed with the condition are not obese, just note that obese is the only recognized cause. Who wants to pay for problems for ugly fat people seems to be just under the surface. Due to this, it is typically harder to service connect than other conditions. The examples I have read, the VA disregards earlier symptoms justifying the condition starting in service out of hand.
http://www.hadit.com/forums/topic/48251-sleep-apnea-granted/ shows an example of someone being granted the connection. Hadit or yuku vets is often a better source for these questions, as they are more focused towards veterans dealing with the system, rather than service members dealing with the system like this board.