Why we need Kernel#gsub?

Ruby
1

I mostly use String#gsub (
Class: String (Ruby 2.1.0) ) . I am
familiar with it also.

But today I found Kernel#gsub (
Module: Kernel (Ruby 2.1.1) ), which I
have never used it.

What is the use-case of this method ?

2

I tried to explore this as below :

(arup~>~)$ echo ‘matz’ | ruby -n -e ‘puts $_.gsub(/./,"*")’

(arup~>~)$ echo ‘matz’ | ruby -n -e ‘$.gsub(/./,"*") ; puts $’
matz
(arup~>~)$ echo ‘matz’ | ruby -np -e ‘$_.gsub(/./,"*")’
matz

Docs is saying - Equivalent to $.gsub…, except that $ will be
updated if substitution occurs. But in my case, it seems $_ has
not been updated.

The reason is String#gsub has been called. Not Kernel#gsub. See
below :

(arup~>~)$ echo ‘matz’ | ruby -n -e ‘p $_.method(:gsub)’
#<Method: String#gsub>

What am I missing in the doc ? How to call Kernel#gsub ?