So there’s two things going on with your project file. They’re both pretty easy to fix though. Let’s have a look at them:
The first thing is the really wide button in your banner area. You can see it here:
Button contents all remain on one line and don’t wrap around to the next line. This is by design. Since it isn’t wrapping it is pushing out width of the entire page on small screens. To fix this, just create a shorter button label. Generally buttons should have smallish amount of text. You don’t want to overdo it.
You next problem is one you already answered for yourself above, and that is you’re using a fixed width for your Gallery stack, as seen here:
Since you’re working with responsive design you don’t want to set a fully fixed width on anything. By doing so you’re telling that element to always be 600 px wide, at all times. If you change that stack back to Fill instead of Fixed width, you can then drop it into a Foundry Container stack and set its Max-Width to to 600 px. This form of Fixed Width that comes in Foundry uses a Max-Width instead of a constantly fixed width, meaning the contents will be 600 px wide until the Container goes below that threshold. When it does, it will begin to size down responsively.
Hope that all helps.