You can remotely install an app by going to the Play Store website on your computer (logged into the same Google account as your phone), selecting an app, and clicking the Install button (which will prompt you to select a device to install to).
I can't find an app called Gadget Guard when I search Google Play. There's a company called Gadget Guard that makes screen protectors, but that shouldn't be an app as well.
Try going to Settings>Security>Device Administrators and see if Gadget Guard is listed there. If so, uncheck it, then go to Settings>Apps>All and see if you can find it there.