Is Redis ReJSON module compatible with Google Memorystore? I want to store JSON in memorystore, can I use ReJSON module?
No you cannot use the ReJSON module because it's currently not supported to use any modules in Google Cloud Memorystore as it is running v3.2.11 and the user modules system was introduced in Redis v4.0. Also in the ReJSON module requirements it stated that it requires Redis v4.0 or above.
Related to this, there is already a public feature request for Google Cloud Memorystore to release v4.0 in which it is stated that:
We are working on releasing 4.0 in Q1 (2019). We will follow up with 5.0 shortly after that.
I cannot confirm it but it can be that after the v4.0 is introduced, you are able to use the ReJSON module.