GitLab email filtering for Gmail, based on their custom headers.

GitLabEmailFiltering.gs

/**

 Setup: 
   - Create a "GitLab Catch-all" label and redirect all GitLab emails to it via normal filtering
     - You can hide it from the message list, so it doesn't clutter up your sidebar
     - Set this catch-all filter to skip the inbox
   - Create issue and merge request labels for each GitLab project
     - This gist uses nested labels like "GitLab/nifty/Issues", but customize to suit your needs
     
*/

function processInbox() {
  var threads = GmailApp.search("label:gitlab-catch-all");
  for (var i = 0; i < threads.length; i++) {
    processThread(threads[i]);
  }
}

function processThread(thread) {
  var newLabel = getLabelForThread(thread.getMessages()[0]);
  var oldLabel = GmailApp.getUserLabelByName("GitLab Catch-all");
  thread.addLabel(newLabel).removeLabel(oldLabel);
}

function getLabelForThread(message) {
  // NOTE: If you wanted to be more precise, you could parse this string to get only the headers,
  // then restrict the rest of the method to search the headers. There's no built-in method to
  // fetch the headers from a GmailMessage. Why?!
  var body = message.getRawContent();
  
  // TODO: map your GitLab orgs and projects onto the project component of your Gmail label
  var project = null;
  if (body.indexOf("X-GitLab-Project-Path: my-awesome-org/my-nifty-project") > -1) {
    project = "nifty";
  }
  
  var type = null;
  if (body.indexOf("X-GitLab-MergeRequest-ID:") > -1) {
    type = "Merge Requests";
  } else if (body.indexOf("X-GitLab-Issue-ID:") > -1) {
    type = "Issues";
  }
  
  // TODO: check project and type for null values, if you're not feeling confident
  return GmailApp.getUserLabelByName("GitLab/" + project + "/" + type);
}