Author's point of view... dbgate have much simpler user interface. dbeaver is powerful, but it has traditional design like other applications made by Java. dbeaver looks like Eclipse, whereas dbgate looks like VS Code. Another point - powering by javascript allows lot of "hacking" - plugins as NPM packages, data export/copy jobs exported as js files, which can be run with nodejs. This cannot be accomplished with Java.

It appears you need to buy the Enterprise Edition of DBeaver to get MongoDB support. And that is a recurring cost, not a one-time cost.

At first glance, Dbgate is faster to start than Dbeaver.