The Air Force is the most strict on this matter. The Air Force allows less than one percent of annual enlistments to be recruits without a high school diploma. The Marines have the next highest standards. No more than five percent of Marine recruits can be GED-holders. The Army allows no more than ten percent each year, and the Navy limits GED enlistments to no more than five to ten percent each year.
There are always many, many more GED-holders who want to enlist than there are available slots, so -- even to be considered -- a GED holder must score much higher on the ASVAB, than a high school diploma recruit.
However, if a recruit has 15 or more college credits, he/she is in the exact same enlistment category as a high school diploma holder.
For more information, see our article about Military Enlistment Standards.