It’s common to installed multiple versions of the same gem. For example, rails with 5.2.7 and 6.1.1, etc.
By default, it uses the latest release.
To specify the old one, just use command like below, which specifies the expected version with underscode _.
$ rails _5.2.6_ new project-in-rails-5.2
Did not go into further if the magic _5.2.6 parsing is provided by gem or rails itself.