Unfortunately, the current version of Gantry is using Bootstrap 2 instead of 3 and what makes this worse is that the default Gantry theme is relying on its own grid system (
not the Bootstrap grids) which is pixel based and not flexible to serve predefined breakpoints by default. although the developers of Rocketheme start to use the Flex-box layouts in their latest themes but they didn't update the default Gantry framework theme which is still bloated with extensive use of textures, gradients, and shadows.
I'm not saying that Gantry framework is worthless, Actually it is a great framework and I was using it since years but it became a bit outdated and you have to tweak it to serve your design needs nowadays.
The Good News is that Rocketheme announces Gantry 5 which expected to be the fastest, most reliable & flexible version of this popular framework. So lets wait for the Rocket Magic :)