You mentioned Gallagher and high frequency.
Gallagher utilizes MIFARE DESFire EV2 or EV3 for their credentials.
If so you are fucked
Use TagInfo
and confirm if it is DESFire
BUT
If you cant read it, that is a very good thing, as
Galaghers older tech, was LF, this is capable of being cloned.
ALSO
They did make dual frequency readers.
Which means there might be a small possibility you could have some success