Hi @phantomroe
As mentioned in this anouncement, Mbed TLS is now maintained under open governance at TrustedFirmware.org.
I would suggest you post your question there, as it doesn’t seem your question is Pelion related.
At a glance, your issue is not a compile time error, but a link time error, so this is not a missing #include in your code. This is because during link time, the gcm symbols were not compiled. Do you have MBEDTLS_GCM_C defined in your configuration?
Regards,
Mbed Support
Ron