wpgraft reads the page as your browser rendered it, every element and the computed styles on each one, at every breakpoint you ask for, and rebuilds it as Gutenberg’s own native elements with the styles in Gutenberg’s own fields. It needs no access to your Squarespace account, no export file and no API key. If Chrome can draw the page, wpgraft can copy it.
Squarespace’s export gives you posts and pages as WordPress-flavoured XML with the design stripped out, which is exactly the half you did not need help with. wpgraft takes the other half: it copies the rendered design, Fluid Engine grids included, and rebuilds it as core WordPress blocks.