Google cares - if they can get a cut of the profits. They allow even scammy malware while blocking FOSS applications. Even recently, they blocked a FOSS chat app.
Conventional wisdom is that only a copyright holder of included GPL code could go after a publisher that breaches the GPL (by claiming copyright infringement). However, recently there's been a lawsuit brought on grounds of breach of contract, which allows any user to be the plantiff.
That case is set to go to trial next month, so I guess wait and see if SFC succeeds and then, if it does, you could sue those app makers yourself and cite SFC v. Vizio as precedent.
Depends what nation you are in and how you obtained it.
Anyone can release software under any licence. As long as they are not breaking the licence they release under. Or the licence they use any 3rd party code is under
I do not think GPL has any rules about claims. Just actual actions. But if they released in under another licence. Then it is possible. (But unlikely). The licence has such rules.
So in most cases. Actual actions or lack of rather then claims. Based on the licence is your only option. And that would mean contact he authors of any included code. Or FSF etc.
Some nations have advertising rules. Depending on how and where it was obtained you may be able to contact their advertising standards association equiv.
But providng for free can often weaken this. Although it is likely far from an absolute excuse to false advertising.
Note that there is not a requirement that they make their source code public, only that they provide source code to users who have received binaries. This means if this app is GPL licensed you can buy it and make a written request for the source code, and they are required by the GPL to provide it.
There is also not a requirement that the binary made available gratis. "Free" in free software or FOSS is about freedom, not price. This is a common misunderstanding of both terms.
where does it say its open source? I do not see this anywhere. what is the stated license?
assuming it does say this somewhere, have you attempted to contact the developer to request the source code? for example here https://app.macoou.com/inquiry What was the result?
if yes to the above and no resolution:
could try reporting via whatever google's mechanism is; "flag as inappropriate" i guess
could contact the SFC https://sfconservancy.org/copyleft-compliance/ they are the only org I am aware of that does this kind of thing as a general activity; I doubt they would be interested in this little hobby project-looking dev
if the dev is using FLOSS code, for example which was published under GPL, and they are not complying with the license in redistribution, then you could notify the devs of the GPL code
if you wish to pursue the matter independently you will need to find about about the dev's local jurisdiction and how to carry out a legal action there. looks like that would be japan.