Yeah it's too bad Google aren't implementing this. I just put together a little JavaScript bookmarklet for this purpose here. Clicking once inserts a CSS style to hide the headers, clicking again removes the CSS and shows them again.
javascript:(function(){var hidecss = "body { margin-left: -45px;
margin-top: -47px; } div.grid-table-container { width: 100%
!important; }
div.native-scrollbar.native-scrollbar-ltr.native-scrollbar-y { left:
initial !important; right: 0px; }
div.goog-inline-block.grid4-inner-container { width: 100% !important;
}"; if (document.querySelector('#hidecss')===null){ var hidescript =
document.createElement('style'); hidescript.type = 'text/css';
hidescript.id = 'hidecss'; hidescript.innerHTML = hidecss;
document.head.appendChild(hidescript); } else
document.getElementById('hidecss').remove();})();
May have to be tuned to other screen sizes but that's the gist of it !