Add `depends_on :gpg`

Library/Homebrew/dependency_collector.rb

@@ -107,6 +107,7 @@ class DependencyCollector
     when :macos      then MinimumMacOSRequirement.new(tags)
     when :mysql      then MysqlDependency.new(tags)
     when :postgresql then PostgresqlDependency.new(tags)
+    when :gpg        then GPGDependency.new(tags)
     when :fortran    then FortranDependency.new(tags)
     when :mpi        then MPIDependency.new(*tags)
     when :tex        then TeXDependency.new(tags)

Library/Homebrew/requirements.rb

@@ -59,6 +59,13 @@ class PostgresqlDependency < Requirement
   satisfy { which 'pg_config' }
 end
 
+class GPGDependency < Requirement
+  fatal true
+  default_formula "gpg"
+
+  satisfy { which("gpg") || which("gpg2") }
+end
+
 class TeXDependency < Requirement
   fatal true
   cask "mactex"