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You might do something like

var codeBoxes = document.querySelectorAll("pre:not(.gml)");
for (var i = 0; i < codeBoxes.length; i++) {
    var codeBox = codeBoxes[i];
    codeBox.classList.add(".gml");
    codeBox.innerHTML = gml_string.renderToHTML(codeBox.innerHTML, "gml");
}

and add some CSS for it

.gmlError { color: red }
.gmlCommentLine, .gmlCommentBlock { color: #008000 }
.gmlKeyword, .gmlCurlyBracket {
	color: #00379B;
	font-weight: bold;
}
.gmlNumber, .gmlString { color: #00f }
.gmlLocal { color: #748 }
.gmlHeaderMeta, .gmlHeaderName { color: #0078AA }
.gmlScript { color: #808 }
.gmlFunction, .gmlMacro, .gmlVariable, .gmlEnum, .gmlEnumField {
    color: #990000;
}

or, for websites with dark background:

.gmlError { color: #f0f }
.gmlCommentLine, .gmlCommentBlock { color: #5B995B }
.gmlMacro, .gmlEnum, .gmlEnumField {
    color: #FF8080;
}
.gmlNumber { color: #FF8080 }
.gmlString { color: #FCF320 }
.gmlKeyword, .gmlCurlyBracket,
.gmlFunction, .gmlScript {
    color: #FFB871;
}
.gmlLocal { color: #FFF899 }
.gmlVariable { color: #58E55A }
.gmlHeaderMeta { color: #A7B }
.gmlHeaderName { color: #77BB5B }