Search queries with greater-than or smaller-than

Hello there,

I am just starting with LDAP, but there is some basic thing that I do not understand and that does not make sense at all:

I have got an OpenLDAP server and it does contain data.

This search returns a bunch of results: ldapsearch -x -v -H ldap://localhost -b ou=something,dc=domainname,dc=com -LLL '(gidNumber=500)'

Yet this returns no results: ldapsearch -x -v -H ldap://localhost -b ou=something,dc=domainname,dc=com -LLL '(gidNumber>=500)'

So I invert it and this returns no results either: ldapsearch -x -v -H ldap://localhost -b ou=something,dc=domainname,dc=com -LLL '(!(gidNumber>=500))'

This simply fails (okay, I knew that, but at least I know the ones above are not syntax errors, because the greater-sign alone is not defined as a filter in RFC 1960 nor RFC 4515): ldapsearch -x -v -H ldap://localhost -b ou=something,dc=domainname,dc=com -LLL '(gidNumber>499)'

Can someone point me in the right direction, please?

Thanks in advance, Tobias