In latest distribution [http://nim-lang.org/download/nim-0.13.0_x64.exe], if you installing MinGW distribution, you will get GDB I386 version (not x64). With this GDB you cannot debug x64 executables. You will get "not in executable format: File format not recognized" error. Is this bug?
gdb.exe [SIZE: 3 598 848 bytes) MD5: 15AFBA60A8861DD1E23CFE39746DC587 SHA-1: E397683412E63D5DFC26B6978B899C9BAB386814 SHA-256: DA925AFE8A0C96452C9A07522E16C5224AB458096A30BCB00BAE102C5D622BC8 SHA-384: 815CF66C8A251CB32D9B92F8CE7D18484031E50662AAC14D0E2F092BEA353EBD82483C907F6760700CC3F1C6EAA6EF2C