Wow, why do people get so defensive about software, for heaven's sake? It's sounds like an Apple v Android flame war on here!
I have tried GPM. I am on my wife's Spotify family plan and I subscribe to Tidal HiFi, so here are my thoughts.
My observations are based on using an LGV10 with some Yamaha EPH100s and a short earphone extender to fire up the high-gain mode on the amp. (Opinions vary on whether this does anything other than boost the volume, but to me the sound is, marginally, better). Also occasionally via a Sony amp and speakers as well.
1. Apps. Taste varies regarding design, and once you get used to how they work they all do the job (although Spotify is better than Tidal in many ways), but I have found stability issues on both Tidal and Spotify. I had real problems with Spotify 6 months or so ago, but it seems better now. Tidal is definitely worse, I have to restart it several times a week. Not a big problem, but annoying. GPM was rock solid for me.
2. PC interface - I compile playlists on a PC and, for me, Spotify is best. Tidal does the job though.
3. Social and sharing. Spotify wins this hands down. It is head and shoulders above other platforms for collaborative playlists and sharing songs. I haven't really tried Tidal for this since all my friends are on Spotify, but it doesn't seem to offer the same level of functionality. If you do this sort of thing a lot Spotify is king.
4. Library. To be honest I cannot see a huge difference between the 3. It looks like they all have deals with the main labels now, and sure there are a few things missing from Spotify (Radiohead's latest, Prince etc) which appear on GPM and Tidal, but in general if I find something missing in Tidal, it is usually absent from Spotify as well and vice versa. Other people's experience may well differ, depending on what they listen to.
5. Last and most importantly, Sound Quality This is the most difficult area to be objective on because there are so many variables, network, hardware and not least the human ear. There is also human bias - if we think or want to believe something sounds better (or worse) then it probably will. My comments are based on having the quality permanently set to the highest quality on all apps in all circumstances (I am usually on Wifi and have 10Gb a month of data so file size is not an issue really), so for example Spotify Extreme and Tidal Hifi and, I stress, this is one flawed human's opinion. So here goes...
GPM sounds fine, if a little flat. Bass can become a bit compressed and it seems to lack a certain richness compared to the other two - particularly when played through speakers. To me the difference is not discernible on my stock Sony earbuds, but is on the Yamahas - for example the bass on The Smiths' "There Is A Light..." doesn't have the same bounce and agility as on the other 2 services.
Spotify sounds a bit fuller than GPM, and in general sounds very good. Often songs sound just as good on Spotify as they do on Tidal HiFi, and, at half the price, it is very good value. If I had to bin Tidal for Spotify then I would be fine with that. There are occasions though when Tidal does make a difference...
Tidal - is it all that? If you have a phone with a Snapdragon DAC then almost definitely no. I couldn't tell the difference on my Sony Xperia. If you have an HTC10 or a V10 like me then maybe. For me, there are some tracks where the difference is clear, and I'll try and explain how. One of my reference tracks is Bowie's 'Heroes'. This is a song that on huge speakers sounds sweeping and epic, but on headphones sounds cluttered with the guitars clashing, the bass sounding crunchy and Eno's synths just getting in the way. With Tidal HiFi, whilst not perfect, there seems to be much better separation and a much wider soundstage which makes a massive difference compared to Spotify. Similarly, Jane Weaver's "The Silver Globe" album sounds a bit thin on Spotify, whereas on Tidal the bass has a bit more kick, the guitar stands out from the swathes of synths adding a bit more dynamism to the whole thing. These differences are subtle, and not consistent, but to my ears they are there.
So what is best? The answer of course is... it depends. In my opinion Tidal at times provides a discernible but mostly slight difference to Spotify, but whether it is good enough to warrant the greater cost and considerably weaker user experience is down to personal preference. I stream about 4 hours of music a day, so £20 a month seems good value to me, but if I tended to listen to locally stored music and used streaming for mainly discovery and commuting then I would say Spotify is perfect - or even GPM if I'm not into sharing and collaborating on playlists.
They're all a heck of a lot better than that old Sony Walkman cassette player us old guys remember having to use for music on the go years ago.