I see these two system variables

Dpact.provider.branch=${GIT_BRANCH}
Dpactbroker.providerBranch=${GIT_BRANCH}

Are they the same?

They are not the same. See https://github.com/pact-foundation/pact-jvm/blob/master/docs/system-properties.md

If I want to enable pending pact is this config enough?

stage('Contract Tests: Provider Verify') {
                    when {
                        expression { PACT_PROVIDER == true }
                    }
                    steps {
                        sh """mvn test -Pcontract-tests-provider-verify \
                        -Dpact.verifier.publishResults=true \
                        -Dpact.provider.version=${GIT_COMMIT} \ 
                        -Dpact.provider.branch=${GIT_BRANCH} \ -->> register branch
                        -Dpactbroker.providerBranch=${GIT_BRANCH} \ -->> enable pending pact
                        -Dpact.broker.consumerversionselectors.rawjson=[{"mainBranch":true},{"deployed":true}]"""
                    }
                }

I imagine so, it looks ok to me.

I am testing it right now. I used to have

pactbroker.providerTags,

but we want to move away from tags.