Put the tag defined in the same file with current buffer most above when there are more than one tags found and one tags found in buffer-file-name.

helm-gtags.el

(defun helm-gtags--exec-global-command (type input &optional detail)
  (let ((args (helm-gtags--construct-command type input)))
    (helm-gtags--find-tag-directory)
    (helm-gtags--save-current-context)
    (let ((buf-coding buffer-file-coding-system) (current-buffer-filename (buffer-file-name)))
      (with-current-buffer (helm-candidate-buffer 'global)
        (let ((default-directory (helm-gtags--base-directory))
              (input (car (last args)))
              (coding-system-for-read buf-coding)
              (coding-system-for-write buf-coding))
          (if (zerop (apply 'process-file "global" nil '(t nil) nil args))
              (progn (let ((global-out (buffer-string)))
                       (erase-buffer)
                       (insert (s-join "\n" (cdr (--map (car it) (--sort (or (s-suffix? (cadr it) current-buffer-filename) (string< (car it) (car other))) (--map (list it (substring it 0 (string-match ":[0-9]+:" it))) (split-string global-out "\n")))))))))
            (error (format "%s: not found" input)))
          (when detail
            (helm-gtags--show-detail)))))))