This is a known issue that should be fixed in master (meaning it will be fixed in the 1.2 release coming up). GitLab and Slack send 'attachments' with their outgoing webhooks which we don't yet support.
The fix currently in for the 1.2 release will stop the incoming webhook from breaking when attachments are present (it will just use the plaintext) and it's on our roadmap to convert Slack's attachments to Markdown so that we're 100% compatible