I've submitted a rebuttal statement for my GCM due to an APFT failure. Passed 2 diag. & a record within 3 months. Can my award be approved?

I'm more concerned about the APFT failure. You are young, you should be able to drink till 0400 wake up smoke a cigarette and pass an APFT without even thinking about it.

Army Regulation 600–8–22 Chapter 4 Page 58

4–8. Disqualification for the Army Good Conduct Medal
a. Conviction by courts-martial terminates a period of qualifying service; a new period begins the following day
after completion of the sentence imposed by the court-martial.
b. Individuals whose retention is not warranted or for whom a bar to reenlistment has been approved under the
provisions of AR 601–280 (specifically for the reasons enumerated in AR 601–280) are not eligible for award of the
AGCM.
c. In instances of disqualification as determined by the unit commander, the commander will prepare a memorandum
stating the rationale for his or her decision. This memorandum will include the period of disqualification and will
be referred to the individual according to AR 600–37. The unit commander will consider the affected individual’s
statement. If the commander’s decision remains the same, the records manager will Web upload the memorandum and
the individual statement for filing in the Soldier’s AMHRR.
d. Disqualification for an award of the AGCM can occur at any time during a qualifying period (for example, when
manner of performance or efficiency declines). The personnel office will establish the new “beginning date” for the
Soldier’s eligibility for award of the AGCM, enter the new date and code on the Soldier’s eMILPO record, and submit
the eMILPO transaction. These procedures do not apply if Soldier is disqualified under the provisions of paragraph
4–8b.

Your GCM should not have been denied for APFT failure. If you were flagged then that is different. Even then, it does not prevent award for the GCM. Refer to AR 600-8-19 under the GCM and to AR 635-200?, for Adverse Actions to provide the regulatory guidance to your chain of command and support channel.

If they deny your GCM based upon the APFT it may be something that they are doing to all Soldiers that fit in that category locally. It may be in your best interest to project your desires regarding reenlistment early, as it wouldn't surprise me if they bar you or deny the reenlistment. So see what can be done to ensure you are viewed in a favorable light.
Appealing the GCM denial is allowed, but ensure you don't fling mud and stick purely to facts of what you have done to overcome the APFT flag.