If I build my application with --gc:orc, the GC_fullCollect, GC_enable, GC_disable are gone.
Then I found there are GC_runOrc*, GC_enableOrc* and GC_disableOrc* in lib/system/orc.nim as replacement, but nim told me they are undclared identifier, how can I call these procs?
"nim told" - was it the compiler itself or the editor? If you compile with --gc:orc in the terminal, then the editor doesn't know that you're using --gc:orc, to fix that you need to create a file like yournimfile.nim.cfg and put that switch inside:
gc:orc