Hello!
I've contacted GMMP support and they're convinced the cause of the issue lies somewhere in my tagging using MP3tag. I've attached a link to the details I've provided them along with images and their responses thus far. Appreciate any help you can provide as this has been a very frustrating problem.
https://gonemadmusicplayer.proboards.com/thread/1584/album-artist-tag-overwritten-info
Thanks!
As guest in that forum I cannot see the screenshots in a readable resolution.
Also, a description of your tagging process would be nice if you suspect that that causes problems.
Please note that the field name for the album artist is ALBUMARTIST.
A screenshot of the extended tags dialogue for a single file would help
A screenshot of the Mapping would also be nice.
Thank you for the speedy reply! Relevant screenshots are attached, if more would help, please let me know. As for my process, I usually start by selecting an individual track from Windows file explorer and then opening with MP3tag via Open With; then, I just update or add info as needed for album art and to the fields for Track, Title, Filename, Album, Artist, Album Artist and Year. In the advanced options, selecting all for APE didn't fix nor did adding a mapping line for ID3v2-Album Artist-ALBUMARTIST. Appreciate your help! 
Only 3 max images allowed at a time, here's an Extended Tags view. This track is from a different album than the one in the screenshot showing Various Artists, but you see I'd like this to group under the Album Artist "Wilco"; currently in GMMP, no Album Artist tags are being read, the player is just duplicating Artist info for all.
I see that these are wav files.
Now it would be a question what the player makes of these files.
As an experiment, you could recode these files to MP3 and copy the tags from the wav original files to the mp3 copies and see whether the player can cope better with mp3 files.
Or in other words: I do not see anything suspicious in the tag fields.
GMMP forum verifies "wav in taglib should support id3v2" but recommends converting to FLAC, will give this a try. Thanks for verifying the settings look OK, it's good to know I don't need to keep messing around with them. 
Good to see you have an option here after identifying the Albumartist issue is when using .wav files. I'm curious to see if this gets resolved after trying a test with converting to a lossless format like FLAC or even lossy mp3. Please provide some feedback after you have a chance to test any of these.
Hi Motley, recognize your avatar from the GMMP thread reporting the same issue as me. Happy to say, though after only 1 trial, converting .wav file to .flac preserved the Album Artist info perfectly - WOOT! 
I'll be converting more files tomorrow, hopefully with same success. Will post an update over in GMMP. Thanks, again, to ohrenkino for noting that the .wav aspect could be causing the issue!
