Provided that everything is working with your remote access IPSEC VPN config / LDAP account unit, the next step to 'enable LDAP authentication' would be to create an access role, bind it to an AD user or group, and add that access role to your access policy. This is assuming the default authentication mechanism of username + password.
By doing this, you will enable the gateway to successfully match a user search against a rule that allows them to connect. I attached an example from my lab where I added a group from AD to an access role.
See admin guide info on:
Access roles:
https://sc1.checkpoint.com/documents/R81/WebAdminGuides/EN/CP_R81_SecurityManagement_AdminGuide/Topi...
Retrieving Information from a User Directory Server:
https://sc1.checkpoint.com/documents/R81/WebAdminGuides/EN/CP_R81_SecurityManagement_AdminGuide/Topi...