Fabrik uses Bootstrap. Gantry4 is only "based upon" Bootstrap (and an old version at that). So, if you want full bootstrap then you will need to add it in using an extension such as this
http://extensions.joomla.org/profile/extension/style-a-design/templating/twbootstrap
. Then you will need to configure the many options in that extension. Even after that you may still need to do some custom CSS to get everything working ok.
I have done this myself on Afterburner2 sites for my clients, so it is possible, but it does take some time to get it all working correctly.
Regards, Mark.
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