Your old template has some weird sizes and that will mean that you have to change the breakpoints more if you want it to look identical (which I actually don't recommend - since the existing ones are industry standard breakpoints).
Next you are using the "home" outline to show your content which has some unqiue styling for the template which won't be required if you want to replicate exactly the existing look of your site. You can improve things though with this CSS:
.section-paddings, #g-top, #g-header, #g-above, #g-utility, #g-feature, #g-expanded, #g-extension, #g-bottom {
padding: 0 !important;
}
Even then though it's still going to require a lot more custom CSS to reverse out the template styling in order to make it look like your old template. I'd suggest that you consider actually using the styling of the template in redeveloping your site rather that trying to replicate your existing site, it will be a far eqasier route to take.
To create a custom CSS compatible with Gantry 5 please read this
http://docs.gantry.org/gantry5/tutorials/adding-a-custom-style-sheet
.
Remember to recompile CSS from base outline too.
Regards, Mark.