As of T219694, MediaWiki responses in Quibble and Fresnel now support Gzip. This makes index.php and load.php responses there more like production and thus Fresnel results more accurate of what end-users will perceive.
However, as part of working on T121730 I noticed that when changing a reference from being served from load.php to a static file, that its size increased notably by about 30%, which suggests gzip isn't applied here.
I confirmed this locally by running the following from a MediaWiki core checkout:
php -d 'output_buffering=Off' -S localhost:9412 maintenance/dev/includes/router.php
And e.g. accessing http://localhost:9412/skins/Vector/images/unwatch-icon.svg, confirming in the browser that it does not have a compressed response.