I currently have set up Grails 2.3.0 and 2.3.11 inside /opt directory. How I avoid re-installing by making GVM recognize them as existing candidates?
Move each of the directories to ~/.gvm/grails/<version> (e.g. from /opt/grails-2.4.3 to ~/.gvm/grails/2.4.3) and symlink those back to /opt... or vice versa.
EDIT for sdkman
sdkman replaced GVM. use ~/.sdkman instead of ~/.gvm