iOS doesn't do external storage very well, whether it be a NAS or SD card or whatever. Pretty much every app has it's own storage, and getting stuff there depends on the app. There is some common storage, like photos (hence always being prompted by apps to give access to your photos), but even that has limitations.
In the case of GaiaGPS it gets it's maps mainly (only?) from mapservers. So unless there is a way to roll your own you are SOL. It's not like Garmin or some others where you buy "a map" and download it; you are actually sending a query for part of a database, in essence, and then that data is sent directly into the app, and, if needs be, saved in the app itself.
Motion X GPS and I think Trimble's work in essentially the same way. Like GaiaGPS, there really aren't any "maps" as in analogs of paper sheets or quads; you are again querying a database for data. Like google maps. And so it also requires access to something more than a NAS. Ditto for iHikeGPS.
If you want to have discrete maps, like quads, or NPS visitor maps, or maps you georeference yourself then try PDF-Maps by Avenza. You can email them to yourself, buy them in their store, or use iTunes to transfer them. If your NAS has a webdav server built in that might work as well (and I assume your "wifi harddrive" is a species of NAS). If they aren't georeferenced PDFs, at least you can view them, but without live coordinates. But you can do that with Goodreader as well. It might also be if you used whatever file sharing app or the utility that came with your NAS to do a "open in..." from that app; dunno. I find it far easier to just download from cell or wifi and save the maps in the app.
I have verified that you can open at least PDF maps from a NAS in the following way:
First, my NAS is a HooToo Tripmate. It runs on USB power, and you can attach a USB drive (2.0) for a NAS. Since the TripMate doesn't have much power itself, I use a 32GB thumbdrive. The HooToo, btw, is about $28 and another $30 for the USB stick. Very small and reliable.
Second, use a desktop to connect to the TripMate via a browser; you find the stick and upload PDF maps. PDF-Maps doesn't georeference maps, so you'd have to do that yourself or use ones already georeferenced (BTW, you can't buy georeferenced PDF maps from Avenza and then transfer them from a phone to use on your desktop).
Third, use an app like FileBrowser to set up a connection to a NAS, the TripMate, and then download the map. With FileBrowser you'll get a little icon you can tap and get the "Open in..." option; it will list everything that'll open PDFs but you want PDF-Maps. Select that, and bam, there's your map.
TripMate has their own iOS browser app, but it doesn't give you the option to download and then open in another app, although you can view the PDF in TripMate's app. It also works as a DLNA device, BTW, so although it's not a map server it does serve up other media to other phones, PS4s, Xboxes, or whatever.
Rob