Can you specify where you found this information about the limitations of google tag manager "will be supporting only for rails 3.0.0 and higher" ?
Did you find a Gem or something for this purpose?
Ruby 1.8.6 reached end of life in 2011 (that was 3 years ago!) You have little excuse for not upgrading to at least 1.8.7 (although 1.8.7 technically reached EOL at the end of July, it is in a phase-out period).
As far as upgrading Rails to 3, that may or may not be a big job depending on your Rails app. However, this is a good example of why staying current with Rails is significant because generally Gems aren't written for older versions of Rails.
while I doing RND on this, I found that google tag manager will be supporting only for rails 3.0.0 and higher version of rails