Ok, I poked around a little, and it looks like Google Photos has a way of importing directly from DropBox via the web site.
Go to Google Photos in Safari, don't let it redirect you to the app. Delete the app temporarily if you have to. Any photos you already have in Google Photos should be safe, since they are stored online. If you are not sure, check them out via the website before deleting the app (again, only delete the app if you can't get to the site with it installed).
Choose Upload and then DropBox from the menu.
Since I don't want to do this, that's as far as I got. You'll probably have to athenticate with DropBox, and possibly select a folder. From there it should be automatic.
Once the upload is done, Google Photo app should start syncing with the online library the next time you open it. I'm not sure what the settings are, but if you don't want a lot of photos stored locally on the iPad make sure you choose wisely. For instance, the Photos app (when used with iCloud Photo Library) has two modes. One where it optimizes the photo library to save space, and the other where it keeps full sized photos on the device. I suspect Google Photos has a similar setting.
Good luck.
Note: if you can't get to the Google Photos site because it keeps redirecting you to the app, you can try a different browser (like Chrome) and see if it works better. I'm not sure if this is going to be a problem, since I don't have the app installed; but it's been known to happen with YouTube and a few other sites that have supported apps.