Problem comes from this commit
https://github.com/discourse/discourse-gamification/commit/d6c5db58f7bb3d683d55602d0cc84e10d2ab214a
Pinning to immediate previous commit did the trick.
In case anyone wants the workaround, define your plugin like this in hooks section of your discourse configuration yaml :
hooks:
after_code:
- exec:
cd: $home/plugins
cmd:
- git clone https://github.com/discourse/discourse-gamification && cd discourse-gamification && git checkout 6a399b4c220f31de62049504d27ff6e50611fbef