Jenkins: Access job/plugin configuration values inside pipeline
This is possible, but you'll have to use scripted Pipelines, not declarative, disable the Groovy sandbox for pipelines, and dig deep into the Jenkins API.
For instance, I wrote a Pipeline job that edits the parameters of a second job which has a full name of &quot;SCA/deploy_to_environment/master&quot;.  Here's a snippet of that code - I'm eliding how to create the &quot;Param&quot; objects in the first line for brevity, this snippet is just to give you an idea of what a similar use case looks like:
def paramProp = new ParametersDefinitionProperty(envParam, pgParam, infoParam)

def job = Jenkins.instance.getItemByFullName('SCA/deploy_to_environment/master')

// There is no &quot;setProperty&quot; - we need to remove and replace.
// removeProperty is safe to run even if no matching properties are set on the job.
// However, this method only removes one property per method call.
// Theoretically, this means the job could accumulate multiple conflicting
// ParametersDefinitionProperty properties.
// TODO: run removeProperty in a while loop to eliminate all matching properties for extra safety.
job.removeProperty(ParametersDefinitionProperty)

job.addProperty(paramProp)

So to fetch the values instead of simply editing them, you would need to instead call getProperty (API doc here), passing the class name of the GitLab Project parameter classes. I suspect the class name you'll need is available somewhere in the source code or the API docs for that plugin, but I'm not sure.

However, I have to ask, why do you want to do this?  What's the big picture here?
It sounds kind of like you want to share a single Jenkinsfile or build pipeline among multiple repositories.  I also have this use case at my job - we need to have one Jenkinsfile shared across nearly 200 repositories.  If this is similar to your need, then consider using Jenkins Shared Libraries.  You can put all of your build pipeline code into a single shared library, then call the shared library in a single line from the Jenkinsfile in your individual repositories.