Hi Alvaro B,
As mentioned by you:
the users that enter via Gateway may have to enter directly into ECC, the issue being that our ECC forces a password change every n-days. In that scenario the user (I mean the person) will eventually have to remember one password for ECC and a different password for Gateway.
Assumed that you've create an RFC destination to the SAP Gateway system on your front-end server and define the trust relationship between the SAP Business suite system (to be the trusting system) and the SAP Gateway system (to be the trusted system). Also I believe CUA Configuration might not be needed in this case, reason being the user ID in Gateway and ERP should be same ID although Password can be different.
Example: When user ID MobUser logon to app, user and password are authenticated in Gateway. From Gateway to ERP, RFC connection is established by user ID MobUser with trusted RFC.
Hope this helps,
Regards,
Pavan G