bundle open opens a gem in an editor. This is useful when you need to look into the source code of a gem that your application depends on.

The documentation above says that

For this to work the EDITOR or BUNDLER_EDITOR environment variable has to be set.

This means this is likely to open up a VIM-variant editor for you. However, if you set VISUAL in your environment, the command will use that instead. For example, to open in Visual Studio Code, you can push the following in your rc file.

export VISUAL='code'

Here has more discussions between VISUAL and EDITOR environment variables.