Now that he’s finished his install series, Ian Grieve is Troubleshooting The Microsoft Dynamics GP 2013 SP2 Web Client: Could Not Establish Trust Relationship
Troubleshooting The Microsoft Dynamics GP 2013 SP2 Web Client: Could Not Establish Trust Relationship | azurecurve
Install, Upgrade & AdminMicrosoft & the GP EcosystemGP 2013
Archived post from 2014. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.