Java heap overwhelmed by unreachable objects

Any explanation as to why the VM is having so many unreachable objects, and is uncapable of collecting them at all?

(Based on our exchange in the comments) it sounds like this is not a traditional memory leak but some piece of logic that is continuously spamming new objects such that the GC struggles to keep up under the current architecture.

The culprit could be for example some API request that is being made many, many times, or else is "stuck" in some erroneous state like the infinite pagination scenario I described. What either situation boils down to is millions of response gson objects (which point to Strings (which point to char[]s)) being instantiated and then becoming eligible for GC.

As I said you should try and isolate problem request(s), then debug and take measurements to decide whether this is a bug or scalability issue on the part of your application or one of its libraries.

Based on your stats listed, I find it hard to believe you have 1.9G of unreachable data. It looks more like a GC Overhead Limit Reached.

Consider

937.746: [Full GC 937.746: [CMS: 512000K->511999K(512000K), 8.8891390 secs] 2047999K->1962252K(2048000K), [CMS Perm : 115302K->115302K(262144K)], 8.8893810 secs] [Times: user=8.89 sys=0.01, real=8.89 secs]

If this is true, then a Full GC releases 85K of data. If you did have 1.9G of unreachable code, you would see 2047999 -> ~300000.

Also

object space 1536000K, 99% 

Implies something was created and stored to in such a way it escaped a method and is now living probably forever.

I would need to see more evidence that you have 1.9G of unreachable data other then simply being told.