I realize that GWT is not the "hippest" tool out there. Most people seem to be using React/Angular/Something-awesome-js. However, I find the concept of GWT definitely appealing (especially having an Android background).
Is there any reason why I wouldn't want to invest in this technology (especially as a junior developer looking to become more employable)?
React is hot for two reasons. It is the new hotness. It addresses a particular set of issues that arise in a particular set of circumstances. Some projects are using React because new and shiny is often fun. Other projects are using it because it is the right tool for a particular job.
Early on, the thing that matters is gaining experience that can be used to make informed judgments in the years to come. Knowing what GWT can and can't do goes toward that. Knowing it's bounds will help determine when React is a better choice.
But coming back, the real choice is "what do I build?" not "How should I build something?" when there's no clear idea about what sort of thing that something is. "A React app" isn't a meaningful description, it could be a toaster or an insurance form.
Good luck.