gcm: partial tag size

[ALaggyDev]

Fix of #500

Shouldn't be a breaking change but I am not sure exactly.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.55%. Comparing base (2751627) to head (a33f7d3).
⚠️ Report is 176 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@## master #501 +/- ##
==========================================
+ Coverage 81.54% 81.55% +0.01% 
==========================================
Files 32 32 Lines 1284 1285 +1 ==========================================
+ Hits 1047 1048 +1 
Misses 237 237 

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[tarcieri]

Thanks!