Can I be denied leave?

As many have already stated here, yes leave is a Right (as defined by Congressional Act) but Commanders have HUGE discretion for approval.

My best advice is to start generating a paper trail. Submit your leave request in writing on the appropriate forms. The AF had a specific form for Leave and/or Liberty. I assume the Army has similar. Submit the form, have the Commander deny in writing and then KEEP THE FORM SECURED! If/when you do end up in a use/lose situation, you'll have the evidence needed to retain those days without them being lost. Not to mention, IF it comes to an IG investigation, you'll have documented proof of denial. Without the forms, all the CO has to say is, "I never received a leave request from the SGT".

Generally speaking, Leave is a RIGHT (as in Legally & Regulatory defined as a Right), compared to Liberty which is a Privilege. The Commander has LOTS of Discretion about when you can take it, based on mission.

That said, submit the papers, and let them deny it. Don't take anyone's word for it. Submit leave papers. The sooner you submit papers, the better the command can deconflict your schedule with others. No single individual is essential to mission, especially if you are giving lots of notice (outside of deployment schedule).

Citation (Army):

http://www.forthoodsentinel.com/news/military-leave-it-s-a-right-not-a-privilege-granted/article_aaa7cfad-dd88-55d5-8bf2-f491019ba6d3.html

"Leave is a right (not a privilege) that is granted by Congress under federal law."

Edit: Added Citation

Before this can actually be answered properly, a little bit more about your unit policy on leave is needed. Secondly, are you getting screwed around - probably, but are you getting screwed - probably not since you are not quite at the use or lose point. Interesting that you mentioned helping for Gunnery, being that you are an 88M (Motor Transport Operator) in what an Armor Brigade of the 1st Infantry Division. Wouldn't that be a normal part of your job? As far as attending the Master Drivers Course, do you not need that for career progression? You say that you were on Rear-end (I am taking that as you were part of the unit's rear detachment) and then spent 7 months on Gate Guard, then supported Gunnery, and now in the Master Drivers Course (local course and not the M9 course, I would venture to guess). My next question - is the unit just back from the deployment or is it still deployed? Next how is the deployment of the BCT to Korea impacting this? You say your morale is drifting; why, because you just came off Gate Guard duty then went directly into some type of training support and then to a Master Drivers Course. If it was Gate Guard duty that wore you down, what about that civilian that was either next to you or on the next over entry point? Oh wait a minute, they chose that and are well paid for that job (??). What is the outlook between now and 6 months from now? A lot of things to think about. How long have you been in this unit?