>Which LDAP server gluu use by default ?
It's OpenDJ.
> It means, Gluu server periodically polls external configured ldap store and syncs this external data store with its internal ldap store. Is this possible using Cache refresh configuration?
Cache Refresh only pulls user data **from** external LDAP directory(s) **into** its own internal directory. It can't sync in reverse direction, by default.
Without further details it's hard to recommend some solution to your cause. For example, if such sync must happen only during some user's login, you could write a custom auth script that does LDAP queries you need. Or, during normal CR cycle, you could also engage your custom CR script which would write each user it pulled into from external LDAP source to some other external LDAP directory.
But if you need it to work completely independently from those features, you will need to write your own app, perhaps; which will pull data from Gluu via SCIM, or via LDAP, and write it to your other directory.