Are you running
ruby -v inside the ruby console? I mean, Bitnami Stacks provides a console where the components inside the stack, paths and variables are loaded by default. If you run
ruby -v without the console, you are obtaining the ruby version installed on the system (it should be a ruby version installed on your windows machine). To use the ruby provided by the Bitnami stack you should load this environment or console.
On Windows, you can do it using the shortcut in the Start menu under "Start -> Bitnami Ruby Stack -> Use Ruby". Once the Ruby console has loaded, execute a Ruby command as you normally would.
For example, in my case (using Linux installer instead of the Windows one, but it should be the same). Without load the environment:
$ ruby -v
ruby 2.4.2p198 (2017-09-14 revision 59899) [x86_64-linux]
this is the system ruby.
If I load the environment:
$ sudo ./rubyconsole
$ ruby -v
ruby 2.3.6p384 (2017-12-14 revision 61254) [x86_64-linux]
now, the ruby version is different, it is 2.3.6 because I installed rubystack-2.3.6-4.
Can you perform these checks and tell us the result?
Carlos R. Hernández