puma is default from gitlab 12.9 and unicorn will be removed from 14.0
gitlab-puma changes is included in puma package.

--- a/Gemfile
+++ b/Gemfile
@@ -170,14 +170,9 @@
 # https://github.com/sharpstone/rack-timeout/blob/master/README.md#rails-apps-manually
 gem 'rack-timeout', '~> 0.5.1', require: 'rack/timeout/base'
 
-group :unicorn do
-  gem 'unicorn', '~> 5.5'
-  gem 'unicorn-worker-killer', '~> 0.4.4'
-end
-
 group :puma do
-  gem 'gitlab-puma', '~> 4.3.3.gitlab.2', require: false
-  gem 'gitlab-puma_worker_killer', '~> 0.1.1.gitlab.1', require: false
+  gem 'puma', '~> 4.3.3', require: false
+  gem 'puma_worker_killer', '~> 0.1.1', require: false
 end
 
 # State machine