Updating for those who may fall into the same issue:
My application not expecting GCEV_EXTENSION events neither SUBSCRIBE or NOTiFY. However those "bad" inbound callers also sending those as I figured.
When going through regular gc_Answer->gc_DropCall process call-leg being cleared properly (at least I'm not noticing anything). Once I'm starting to make early reject (at OFFERED state). those GCEV_EXTENSION events starting to occupy legs forever and causing system degradation as described in docs.
So final solution is to make early reject + always check for GCEV_EXTENSION events and process them regardless if they are expected or not.
Thanks Jeff and Leonid for ideas and help.