From the RCA website:
Non-GPS Conversions
While the RCA encourages its members to use GPS, it recognizes that some may choose not to, in their role as the local rabbinic authority. Such rabbis recognize that other rabbis are similarly free to not recognize conversions from outside GPS in their jurisdiction.
Even the RCA recognizes someone who underwent an “Orthodox” conversion can fairly and halachicly have that conversion not recognized as valid within Orthodoxy, if the other rabbis deem it problematic.