Gosh this was a trivial one and it took me way too long to figure it out.
Solved the issue by creating a file named external-projects.properties in the root folder of the parent project with the relative or full path to the dependency project folder.
To be precise, for me the folders tree ends up like this:
git-folder/
|__parent-project/
|__external-projects.properties
|__build.gradle.kts
|__rest (src/main, etc)
|__dependency-project/
|__build.gradle.kts
|__rest (src/main, etc)
And the contents of the external-projects.properties file:
dependency-project = ../dependency-project
The dependency project must also be in the build path of the parent project (done by "Right-Click project > Build Path > Configure Build Path" and adding it in the "Projects" tab).
After that, a "Right Click > Gradle > Refresh Gradle Project" did the trick.