What is the maximum value for a gid in OpenLDAP

the GidNumber is unlimited.

The GID number is defined as an integer in the schema definitions

attributetype ( 1.3.6.1.1.1.1.1 NAME 'gidNumber'
    DESC 'An integer uniquely identifying a group in an administrative domain'
    EQUALITY integerMatch
    SYNTAX 1.3.6.1.4.1.1466.115.121.1.27 SINGLE-VALUE )

According to RFC 4517 LDAP: Syntaxes and Matching Rules

3.3.16. Integer

A value of the Integer syntax is a whole number of unlimited
magnitude. The LDAP-specific encoding of a value of this syntax is
the optionally signed decimal digit character string representation
of the number (for example, the number 1321 is represented by the
character string "1321"). The encoding is defined by ...

Since it is a character string and not a number you don't have the OS determined maximum size for a signed integer and the GID number in LDAP is unlimited.

On the other hand, your OS support and PAM libraries may be limited to 65534.