It looks to me that you will need to adapt the Gallet-helmet to your existing portable radio connection.
If you compare the connector-PIN designations (earphone, microphone on which pin) from both helmet-connectors, also taking into account possible different impedance values the reason for the problem can be isolated.
The technical description of your mobile radio will give you information about the required data to connect external earphone & mike.
The data of the Gallet-helmet shall be compared to match with this data.
Maybe an adapter-box to match with the Gallet-helmet will do the job.