Gemified in 2.7

The following libraries were made standard gems with Ruby 2.7:

Default Gems

Gem Description Resources
benchmark Benchmark utility: Measures and reports the time used to execute code RubyGems, GitHub, CRuby, RDoc
cgi c Support for CGI (Common Gateway Interface) RubyGems, GitHub, CRuby (1), CRuby (2), CRuby (3), RDoc
delegate Provides three ways to delegate method calls RubyGems, GitHub, CRuby, RDoc
did_you_mean Patches error messages to suggest correct spelling of methods/classes/variables RubyGems, GitHub, CRuby (1), CRuby (2)
getoptlong GNU getopt_long() style command-line option parsing RubyGems, GitHub, CRuby, RDoc
net-pop Support for POP3 (Post Office Protocol) RubyGems, GitHub, CRuby, RDoc
net-smtp Support for SMTP (Simple Mail Transfer Protocol) RubyGems, GitHub, CRuby, RDoc
observer Implementation of the observer pattern, a way to let interested other objects know o an objetc's updates GitHub, CRuby, RDoc
open3 Simple spawning of child processes RubyGems, GitHub, CRuby, RDoc
pstore Transactional file storage for Ruby objects RubyGems, GitHub, CRuby, RDoc
readline If available, readline-ext will be loaded, or (Ruby-only) reline will be used. RubyGems, CRuby, RDoc
readline-ext c Interface to GNU Readline and NetBSD Editline RubyGems, CRuby, RDoc
reline Ruby-only implementation of GNU Readline / NetBSD Editline RubyGems, GitHub, CRuby
singleton Mixin for Ruby classes that should only have one instance RubyGems, GitHub, CRuby, RDoc
timeout Auto-terminates code blocks after the time limit is reached GitHub, CRuby, RDoc
uri URI/URL manipulation GitHub, CRuby (1), CRuby (2), RDoc
yaml require 'yaml' will load the psych default gem, yaml/dbm is a wrapper around DBM using YAML serialization, and yaml/store lets you use pstore with YAML GitHub, CRuby (1), CRuby (2), CRuby (3), RDoc (1), RDoc (2), RDoc (3)

Removed from Ruby

Gem Description Resources
cmath Trigonometric and transcendental functions for complex numbers RubyGems, GitHub, CRuby, RDoc
e2mmap Abstraction around exception handling RubyGems, GitHub, CRuby (1), CRuby (2), RDoc
scanf Pure-Ruby scanf implementation RubyGems, GitHub, CRuby, RDoc
shell Provides a Ruby interface for interacting with the shell RubyGems, GitHub, CRuby (1), CRuby (2), RDoc
sync Two-phase lock with a counter for multi-threaded code RubyGems, GitHub, CRuby, RDoc
thwait Waits for threads to finish RubyGems, GitHub, CRuby (1), CRuby (2), RDoc

Bundled Gems

No bundled gems were introduced in 2.7.

Removed from Ruby

Gem Description Resources
did_you_mean Patches error messages to suggest correct spelling of methods/classes/variables RubyGems, GitHub

More Standard Gems

Fork me on GitHub