Yes & No.
GAO isn't definitive.
It may prevent implantation of a fertilized egg.
If you believe "life begins at conception", you can argue the woman hasn't really conceived until the fertilized egg is well ensconced in her womb, therefore no abortion.
If you believe, as the newly passed Kansas law sitting on Gov. Brownback's desk says, "Life begins at fertilization" , then preventing implantation would be an early abortion.
Plan B is not the same a RU486, (which I always thought should be pronounced "Are You For Aiding Sex". Just had to get that in there.)