Are you saying you need to gc_DropCall right after GCEV_OFFERED? If so, the app MUST specify a reason for rejection, if not, by default the stack would try to send BYE, which is not allowed in this state, and therefore this message, being allocated in GC libs, is never released, since there is no response from SIP network. This may cause a leak (a bit speculating, since didn't see the logs, but it is possible).
If you need to reject an inbound call in OFFERED state (before accepting it), you need to specify one of the cause code with gc_DropCall. Please refer ch. 11 at www.dialogic.com/.../globalcall_for_ip_hmp_v12.pdf for a list of valid codes, pick what seems to be relevant, for example, PEC_SIPReasonStatus403Forbidden. See the whole list in ch 11.5 at the location above