Update 2

Ok, installed now the 32-bit nvidia libraries and it starts now. Got help from here.

Also found out that it will start when you delete/rename the ~/GOG Games/X3 Reunion/lib folder. And you can keep all files in the ~/GOG Games/X3 Reunion/game/lib/ folder but we had to switch out the libz.so.1 with the previous compiled one.

I will write a detailed answer in the answer section to this now.

So, I put the libz.so.1 into the ~/GOG Games/X3 Reunion/game/lib/ folder, but renamed the previous libz.so.1 to libz.so.1.old.

I would then just install the .6i86 ones...

So, I put the libz.so.1, which I compiled from source like @aphid showed it, into the ~/GOG Games/X3 Reunion/game/lib/ folder, but renamed the previous libz.so.1 to libz.so.1.old.

I would then just install the .i686 ones...

Small update

So, I put the libz.so.1 into the ~/GOG Games/X3 Reunion/game/lib/ folder, but renamed the previous libz.so.1 to libz.so.1.old.

then I got:

./start.sh 
Running X3: Reunion
Language detected: English
./X3R_config: symbol lookup error: /lib/libxcb-shm.so.0: undefined symbol: xcb_send_request_with_fds

which lead me to:

this

where someone had the same issue, but for X3 Terran War.

Well yeah, it's an other game, but the same series, so I thought the following might work:

Solution there was to delete the lib folder in the root folder of the game (that one next to docs, game, start.sh.,...) and keep in the lib folder located in games only with:

libavcodec.so.53
libavformat.so.53
libavutil.so.51
libbz2.so.1.0
libcrypto.so.1.0.0
libGLEW.so.1.13
libswscale.so.2

Of course I saved the ~/GOG Games/X3 Reunion/lib and ~/GOG Games/X3 Reunion/game/lib/ before doing this

which got me:

./start.sh 
Running X3: Reunion
Language detected: English
libGL error: No matching fbConfigs or visuals found
libGL error: failed to load driver: swrast
X Error of failed request:  GLXBadContext
  Major opcode of failed request:  153 (GLX)
  Minor opcode of failed request:  6 (X_GLXIsDirect)
  Serial number of failed request:  107
  Current serial number in output stream:  106

which lead me with a search for failed to load driver: swrast to this:

Answer#6

So maybe 32-bit-nvidia-libs might help, or would they conflict with the current 64-bit-libs? X3: Reunion is a 32-bit only game afterall.

Currently installed ones which are 64-bit packages:

akmod-nvidia-340xx
kmod-nvidia-340xx-5.11.11-100.fc32.x86_64
kmod-nvidia-340xx-5.11.12-100.fc32.x86_64
kmod-nvidia-340xx-5.11.14-100.fc32.x86_64
nvidia-texture-tools
xorg-x11-drv-nvidia-340xx
xorg-x11-drv-nvidia-340xx-kmodsrc
xorg-x11-drv-nvidia-340xx-libs
xorg-x11-drv-nvidia-libs

I would then just install the .6i86 ones...

Or would this option be a dead end?