The gc.conf and the grok-custom-patterns addresses the gc message log format below:

2018-03-28T19:16:22.094+0000: 1209276.240: [GC pause (G1 Evacuation Pause) (young), 0.0197488 secs]
   [Parallel Time: 11.1 ms, GC Workers: 28]
      [GC Worker Start (ms): Min: 1209276240.8, Avg: 1209276241.5, Max: 1209276242.2, Diff: 1.5]
      [Ext Root Scanning (ms): Min: 0.4, Avg: 1.2, Max: 1.9, Diff: 1.5, Sum: 33.4]
      [Update RS (ms): Min: 1.6, Avg: 2.0, Max: 2.9, Diff: 1.2, Sum: 55.9]
         [Processed Buffers: Min: 1, Avg: 3.3, Max: 11, Diff: 10, Sum: 93]
      [Scan RS (ms): Min: 1.0, Avg: 1.8, Max: 2.1, Diff: 1.2, Sum: 50.8]
      [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.1]
      [Object Copy (ms): Min: 4.8, Avg: 4.9, Max: 5.0, Diff: 0.2, Sum: 136.5]
      [Termination (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.2]
         [Termination Attempts: Min: 1, Avg: 1.1, Max: 2, Diff: 1, Sum: 31]
      [GC Worker Other (ms): Min: 0.0, Avg: 0.2, Max: 0.3, Diff: 0.3, Sum: 4.5]
      [GC Worker Total (ms): Min: 9.2, Avg: 10.1, Max: 10.9, Diff: 1.6, Sum: 281.5]
      [GC Worker End (ms): Min: 1209276251.4, Avg: 1209276251.5, Max: 1209276251.7, Diff: 0.2]
   [Code Root Fixup: 0.1 ms]
   [Code Root Purge: 0.0 ms]
   [Clear CT: 1.6 ms]
   [Other: 6.9 ms]
      [Choose CSet: 0.0 ms]
      [Ref Proc: 1.8 ms]
      [Ref Enq: 0.0 ms]
      [Redirty Cards: 2.6 ms]
      [Humongous Register: 0.0 ms]
      [Humongous Reclaim: 0.0 ms]
      [Free CSet: 2.1 ms]
   [Eden: 594.0M(594.0M)->0.0B(593.0M) Survivors: 20.0M->21.0M Heap: 875.1M(1024.0M)->282.7M(1024.0M)]
 [Times: user=0.20 sys=0.10, real=0.02 secs]