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I own CotND on steam and GoG, on GoG there are i386 libraries listed as required for the game to run, and on Steam, the game is listed as requires 64 bit.
I have a 64 bot only Linux install, that steam runs on, and I can play CotND via steam AND via GoG.
It seems like the game runs fine, with audio without the libraries installed.
It appears like the requirements should be updated, unless i'm missing something.
This was going to stop me from buying the GoG version of Synchrony, now assuming it also runs on amd64 natively

File Info:
~/GOG Games/Crypt of the NecroDancer$ file game/NecroDancer64/NecroDancer.x64
game/NecroDancer64/NecroDancer.x64: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/, BuildID[sha1]=a067c4c58b389a503b7076e99c75d13713c2375c
Oh, wow. I'm gonna be optimistic and hope that's the intention and just incorrect/old information on the store page. I mailed and asked whether there would be a 64-bit build since 32-bit libs was no longer supported on my os distribution.

you can get some more clues to what the binary wants to link dynamically with the `ldd` and (for a running process) `pldd` tool. Just dont use this approach for investigating potentially malicios binarys, as it's not completely done 'offline/cold', see the manual page for ldd "Security." (pldd is for a running process, so that narrows it down to stuff you trust running already)

I'll most likely buy it then, just being able to play the old game on modern OS is well worth it in my opinion :)
Yes. The new Linux version is 64-bit only on GOG.