GPS spoof disaster: What should we do?
We play Pokemon GO in Ann Arbor, Michigan, USA.
Currently, we are under the fire of GPS spoofer(s) for a month. They did not put Pokemons in gyms after attack, and so we cannot identify the names of the GPS spoofers. The attack is city size. I chat people around there and the trainers are loosing the interest in Pokemon GO. Does anyone have some idea how to deal with this? I reported it to Niantic three times. Other users did, too. There is no improvement and getting worse.

Answers
I see. Well... I dunno mate. You can't prove to Niantic otherwise without using that application and you can't use that screenshot because of that application. I also 'had' a similar problem a couple of months ago. I simply reported them. Haven't seen them since. Not sure if they got banned or simply got bored of the game. However, unlike your case, they're pretty dumb. After taking down the gym, they showcased their regional exclusive Pokémons.
Honestly, I have no idea of the magnitude of their actions. I only happen to witness "them" taking down 4 gyms about a couple of blocks away from each other. All are visible in the map at the same time. What I did was take a picture of the surrounding area, take a screenshot of the gym battle animation and another screenshot of the trainers afrer they put their mons. In your case though, since they aren't really putting Pokémon, it's gonna be difficult to tell who they really are.
The only thing you can do is camp out and try to catch them in the act. Even if they are spoofing, they still have to attack the gym so this would be the only chance to put a name to the person doing this. Unfortunately this may be a waste of time because he probably has multiple accounts. If you are in communication with others in the area, start a group chat or social media group. Figure out what time he does this and have multiple people at different gyms to document/screen shot/timestamp the takeover. If he is spoofing maybe you could prove that there is no physical way to get to each gym in time. I don’t know why people take down gyms just to leave them empty. In fact, I feel kind of bad for them. The amount of effort/time it takes for almost no reward that is really sad…
All you can do I think is just make them waste time. Get with your friends and all bubblestrat/gastlystrt and stick 10 blisseys and chanseys everywhere...
They spoof so they won't run out of resources, but you can still waste their time taking their gyms. If it's a bot, then you are out of luck.
Eventually, they would get bored since it's pretty boring and with a mass of people doing this, it won't be that revive intensive and training is fast if multiple people hit the bubbler mon or better yet, use multiple bubble mon and take the thing to level 10 in 10 minutes...
Help!
This is a recent development for this city-size GPS spoof attack. The attack is still going on with wiping out all Valor teams from the indicated gyms. The attack seems to continue until all Valor trainers extinct in the city. No improvement and no help by Niantic. Please let me know the names of cities under gigantic GPS spoof attack if they exits like this.