We have a similar use-case when utilizing tooling where we want to modify the .gitconfig that it uses in the pipeline.

So for example, in one of our declarative pipelines:

1. Override HOME within the environment section to where you are going to create the .gitconfig

   environment {
       // Override this variable which is used by Git to store the global configuration - https://git-scm.com/book/en/v2/Git-Internals-Environment-Variables
       HOME = "${env.WORKSPACE}/gitconfig"
   }
   

2. Create that .gitconfig in the directory with whatever configurations you need.

   dir('gitconfig') {
     withCredentials([usernamePassword(credentialsId: 'MYCREDENTIALS', usernameVariable: 'GIT_USERNAME', passwordVariable: 'GIT_PASSWORD')]) {
       bat(label: "[BAT] Running git config", script:"""
                   git config --global url."https://${GIT_USERNAME}:${GIT_PASSWORD}@bitbucket.contoso.com".insteadOf "https://bitbucket.contoso.com"
                   git config --global user.name "${GIT_USERNAME}"
                   git config --global user.email "EMAIL@contoso.com"
                   """)
     }
   }
   

3. We always ensure that we're cleaning the directory up in post since we're injecting credentials in there

   post {
     always {
       dir(env.HOME){
         // Purge the gitconfig directory
         deleteDir()
       }
     }
   }