Hi,
few general tips:
Please do all those steps - step by step !
0) Backup
1) Disabe then Uninstall old plg_gkextmenu plugin (!)
2) Upgrade to J3.3.4
3) Re-install template to new version, for J3.x
4) Re-install all gk modules, I mean : mod_image_show_gk4, and mod_news_pro_gk5 - with versions for Joomla 3.x
5) Install plg_gkextmenu and enable plugin which you can take from gk_rockwall_rest_files_J!3 or other package
6) Now add your changes - customization again ,
or
you can try to upload/replace template files from back-up (by do it one by one, then test)
- so there is chance that will work and you will save lot's of time.