Gem is a one binary file library. You hane to import the lib before trying to create an object from it.
Try putting a [declare -path /path/to/Gem -lib Gem] object in your patch.

Do you have any suggestions I could try out? I think Ofelia is a great project, and I'd really like to help with its development/maintenance if needed. There are plenty of stuff about the compiling process that I don't know though.

I can't find any of the equivalent files of the .dll files anywhere. I guess these are the ones you are talking about?

These are the contents of the .zip file you mention:

abs/                  CHANGES.txt                    examples/     LICENSE.txt  ofelia-object-help.lua  ofelia_textwindow.tcl   opencv_core4100.dll  opencv_features2d4100.dll  opencv_imgproc4100.dll    README.md
assimp-vc142-mt.dll  classesAndGlobalFunctions.txt  libxml2.dll  ofelia.dll   ofelia-object-help.pd   opencv_calib3d4100.dll  opencv_dnn4100.dll   opencv_flann4100.dll       opencv_objdetect4100.dll  stable-diffusion.dll

Are these supposed to be located in the ofeliaExtendedLib directory? Because, all there is in there, is already in the directory of the ofelia.pd_linux Pd external directory too.

Oh, I followed the instructions, (which are probably forked from ofxOfelia?) where it writes that the directory needs to be named ofxOfelia. I renamed it to ofxOfeliaExtended and make now worked. I copied ofelia.pd_linux to my externals folder, but Pd can't create the object. Trying to load the library with [declare -lib ofelia], I get the following errors:

load_object: Symbol "ofelia_setup" not found in "/home/alex/Documents/Pd/externals/ofelia/ofelia.pd_linux"
load_object: Symbol "ofelia_setup" not found in "/home/alex/Documents/Pd/externals/ofelia/ofelia.pd_linux"
load_object: Symbol "ofelia_setup" not found in "/home/alex/Documents/Pd/externals/ofelia/ofelia.pd_linux"

It turns out that the build instructions are there. For some reason, I thought that the instructions had only to do with the OF addon and not the Pd object. I tried to compile the Pd object, and got this error:

../../../addons/ofxOfelia/src//../libs/ofxImGui/src/BaseTheme.h:3:10: fatal error: imgui.h: No such file or directory
    3 | #include "imgui.h"

I looke for ofxImGui and found this, which I downloaded and put in the addons directory, but I'm still getting the same error.
I'm on Debian 12, with OF 0.12.0.

If you want to do shell stuff, just install the command external. It's up on deken (Help -> Find externals), just look for "command".

@FFW not really, the [sel 0] has to connect to [f ], like @porres did it. But [mousefilter] is indeed the object for this job. Didn't have that in mind.

@FFW I think you got it wrong, unless [mousestate] has changed with a new release. The left-most outlet of [mousestate] outputs the click state, and the second from left outlet is the X position. If you feed that to [f ], then the latter will output the X position of the mouse, not the slider value. @porres solution is correct, though if we want to take into account the "when I move the slider" part of the OP, then a [change] after [f ] is necessary, like @FFW did.

Try [command], which is an actively maintained replacement of [shell]. [shell] is no longer maintained, and that could be the issue.