I want to cancel my last push, because a pushed the app/build folder changes Is it possible to revert a push or the last push in Gitlab?
Either you can delete the commit locally and use force push or create a revert commit via GitLab UI (GitLab 8.5 or newer):
Select your commit in the history you want to revert and click on the revert button:
There will be a modal dialog. You can choice if you want to create a Merge Request or directly commit it to the default branch: