The gstatic loader is our own creation, used only with Google Charts, so we can change things if need be.
I understand what you mean about the css loaded by Google Charts overriding your previous override of the Google Closure css. However, I hope that what I am suggesting still applies. You'll have to make your overrides be more specific to your context than what Google Charts specifies in its css. The more specific rules should win. Something like this:
<body class="mypage">
<div id="chart"></div>
<div id="some_google_closure_thing">...</div>
</body>
Then your css rules can be like this:
.mypage .goog-closure-thing {
/* my styles should override */
}