The title pretty much says it all, but just to clarify:
Redis afaik has a performance metric mem_fragmentation_ratio, giving the ratio of memory used as seen by the OS (used_memory_rss) to memory allocated by Redis (used_memory).
Is it possible to find this metric (or calculate it using other existing metrics) in Cloud Monitoring?
Unfortunately the answer is no. Only the GCP support team is able to verify this metric with their internal tools :(, nevertheless, I found this Public Issue tracker that is requesting to add this metric to monitor the instance. I suggest you do a comment and start the issue so you can get notifications about this :)
on the other hand, about the calculation, I found this link. It seems that the formula is:
MemoryFragmentationRatio= Used_Memory_RSS / Used_Memory
With the command "info all" you can get these values.
I really hope that the aforementioned information helps you.