As I read the AA FAQ on freebie MCE, it is available only to AA elites on MileSaver. This does not include OW equivalents as those are specifically referred to in sections where OW status counts. As an AA Gold, OP would be able to select MCE if still available at T-24.

Q: Can I purchase the new seat product if I am traveling on an AAdvantage MileSAAver Award ticket or Business Extra PlanAAhead ticket?

A: Elite status members may select Main Cabin Extra seats when traveling on MileSAAver or PlanAAhead Award tickets. However, Main Cabin Extra is not available for purchase by non-elite customers traveling on MileSAAver or PlanAAhead Award tickets.