Umbrella strollers can be gate checked, all others should be checked at the counter, still free of charge.
The wording on the AA site, says that umbrella strollers can be taken on the plane and stored in the overhead bin, and that other collapsible strollers under 20 lbs. can be gate checked, but all others are to be checked at the counter. I think OP is asking about the collapsible strollers under 20lbs. I haven't traveled with AA since the change, but I'd be interested to know as well. It seems from their website that as long as the stroller is under 20 lbs. and collapsible, that you'd be able to gate check it.
ETA: From the AA website:
Stroller Acceptance For a Ticketed or Lap Child
Stroller Type Features
Umbrella Stroller
AA highly recommends using this type of stroller
May be taken onboard the aircraft as long as it fits in the overhead bin
Exempt from carry-on allowance and baggage charges
May be checked at the gate
Fully Collapsible Stroller Weighing Less Than 20 lbs.
May be checked at the ticket counter
May be checked at the gate
May not be carried on the aircraft
Exempt from baggage charges
Large Stroller or Non-Collapsible Stroller or Heavy Stroller Weighing Over 20 lbs.†
Strongly discouraged to avoid damage. Facilities were not designed to accommodate these types of strollers
May be checked at the ticket counter†
May not be carried on the aircraft
Exempt from baggage charges
Please note: American is not responsible for damage to any stroller not properly packed in original packaging.
†Effective June 1, 2011, large, jogging, non-collapsible strollers, or those over 20 lbs will no longer be accepted at the gate and therefore must be checked at the ticket counter. There is no charge as long as a child or infant is traveling.
http://www.aa.com/i18n/travelInformation/specialAssistance/childrenTraveling.jsp#strollerdisclaimer