You can define pipeline for merge_requests in .gitlab-ci.yml
However, I want to trigger a pipeline run from a mirrored GitHub Pull Request.
When I create PR, there is no associated pipeline run in GitLab

trigger_build:
stage: build
variables:
TOKEN: xxxxxxxxxxxxxx
script:

• apk add --update curl
• “curl -X POST -F token=${TOKEN} -F ref=master https://gitlab.com/api/v4/projects/15799370/trigger/pipeline”

only:

• master