In a new small branch, set the "version" field in app/package.json (not the top level package.json) to the new version. Create the merge request and merge to dev.
Create a merge request to master, describing the changes since last version. Merge it, then tag it using Repository -> Tags. Gitlab CI will use the version number you just set to create and publish the packaged new version to Snap Store and Bintray.
The publish job will take about 20 minutes. After that, look for the new version on Bintray and Snap Store.