If your gown is produced under the ISO 13485:2003 and EN ISO 13485:2012/AC:2012, these standards are not valid any more. Ther eis new standard ISO 13485:2016 and manufacturers of the medicald evices should be in complaince with this version of the standard.
FDA is responsible for protecting public health by regulating human drugs and biologics, animal drugs, medical devices, tobacco products, food (including animal food), cosmetics, and electronic products that emit radiation. Medical devices for specific medical conditions have to go through an approval process before getting to market (by checking the documentation, production process, performance and safety charactesristics).
„FDA approved“ means that agency has determined that the benefits of the product outweigh the known risks for the intended use. After the FDA approval, and only after it, it is possible to put the medical device on the market.
ANSI/AAMI PB70 - Class 3 Liquid barrier performance and classification of protective apparel and drapes intended for use in health care facilities" is a standard that descibes how gown must be constructed, and to which test it must respond. This standard can not be compared to the ISO 13485 because those two are different kind of standard.
AAMI PB70 is a technical standard, and ISO 13485 is a quality management standard. This means that manufacturer need to have defined processes and management system according to the ISO 13485, and that gown need to be constructed and produced to correspond to the AAMI PB70.
Level of protection for the gown depands on the construction of the gown, their intended use, type of the material.