If I install GM seed in my iPhone 4 now, can I directly upgrade to a future 5.1? Or will I need to do GM -> Final -> 5.1?
Usually, but not always. The GM can sometimes be a few builds older than the final. The things that change are usually private function APIs that don't effect third party apps.
Wirelessly posted (Mozilla/5.0 (iPhone; CPU iPhone OS 5_0 like Mac OS X) AppleWebKit/534.46 (KHTML, like Gecko) Version/5.1 Mobile/9A334 Safari/7534.48.3) If the build is different iTunes will tell you to update if it doesnt no need to restore
They started installing the GM on devices at the factory the same day it was released to developers. That means its on regular devices in the field, so updates will be standard from here forward.