diff --git a/Library/Homebrew/rubocops/patches.rb b/Library/Homebrew/rubocops/patches.rb
index 272e7c8311..79ffc873c6 100644
--- a/Library/Homebrew/rubocops/patches.rb
+++ b/Library/Homebrew/rubocops/patches.rb
@@ -56,8 +56,10 @@ module RuboCop
             problem "GitHub patches should end with .patch, not .diff: #{patch_url}"
           end
 
-          if regex_match_group(patch_url_node, %r{.*gitlab.*/commit/[a-fA-F0-9]*\.diff})
-            problem "GitLab patches should end with .patch, not .diff: #{patch_url}"
+          # Only .diff passes `--full-index` to `git diff` and there is no documented way
+          # to get .patch to behave the same for GitLab.
+          if regex_match_group(patch_url_node, %r{.*gitlab.*/commit/[a-fA-F0-9]*\.patch})
+            problem "GitLab patches should end with .diff, not .patch: #{patch_url}"
           end
 
           gh_patch_param_pattern = %r{https?://github\.com/.+/.+/(?:commit|pull)/[a-fA-F0-9]*.(?:patch|diff)}
diff --git a/Library/Homebrew/test/rubocops/patches_spec.rb b/Library/Homebrew/test/rubocops/patches_spec.rb
index 6a7db5908f..1631d076df 100644
--- a/Library/Homebrew/test/rubocops/patches_spec.rb
+++ b/Library/Homebrew/test/rubocops/patches_spec.rb
@@ -181,7 +181,7 @@ describe RuboCop::Cop::FormulaAudit::Patches do
         "https://github.com/uber/h3/pull/362.patch?full_index=1",
         "https://gitlab.gnome.org/GNOME/gitg/-/merge_requests/142.diff",
         "https://github.com/michaeldv/pit/commit/f64978d.diff?full_index=1",
-        "https://gitlab.gnome.org/GNOME/msitools/commit/248450a.diff",
+        "https://gitlab.gnome.org/GNOME/msitools/commit/248450a.patch",
       ]
       patch_urls.each do |patch_url|
         source = <<~RUBY
@@ -225,7 +225,7 @@ describe RuboCop::Cop::FormulaAudit::Patches do
           EOS
         elsif patch_url.match?(%r{.*gitlab.*/commit/})
           expect_offense_hash message: <<~EOS.chomp, severity: :convention, line: 5, column: 8, source: source
-            GitLab patches should end with .patch, not .diff: #{patch_url}
+            GitLab patches should end with .diff, not .patch: #{patch_url}
           EOS
         # rubocop:disable Layout/LineLength
         elsif patch_url.match?(%r{https?://patch-diff\.githubusercontent\.com/raw/(.+)/(.+)/pull/(.+)\.(?:diff|patch)})