I can answer this for the current Linux version (Installer: enter_the_gungeon_2_1_9_33951.sh).
It has both, a 32 bit and a 64 bit binary - which is quite common.
file EtG.x86_64
EtG.x86_64: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.24, BuildID[sha1]=17c25933fc9fb8dbc515464b8a2b72318846fa83, stripped

OK, I looked at the Windows download:
Enter the Gungeon 2.1.9 364 MB
Enter The Gungeon (32-bit) 2.1.9 343 MB
So there are both versions, too.

And macOS has no requirement given on the product page - so it should be 64 bit, too.

Generally speaking it is a pity that GOG does not mark 32 bit trash with a red banner.
There are product pages saying that 32 bit libs are necessary on Linux even though it is a 64 bit binary ...
with no need to change anything after GOG support ...

32 bit code needs the compatibility mode on current CPUs (AMD after 2003, Intel after 2004; and
Linux supported x86-64 architecture in long mode even before: Linux 2.4 in 2001),
so 32 bit SW should have vanished about 2008 in my opinion.
So Apple was not early to drop 64 bit support with macOS 10.15 released 10/2019
(especially when intending to leave x86 platform).
And Ubuntu/Canonical making a first guess to drop it with 20.04 LTS which was just postponed ...
But technically it is really weird to use such code in 2020 (and even release new games as 32 bit only right now)!

So asking to get a qualified answer is the only way to make sure.

I hope this helps.