big change of my fork ,if you want ,you can merge it , just open a pull request 

  hightpoint:
  1. add helm-gtags-tag-location-list option,
     support multiply tag locations.
  2. replace all call-process-shell-command with call-process
  3. add function helm-gtags-complete
     help completing symbol at point when writing code.
  4. can add helm-source-gtags-files to helm-for-files-preferred-list