rumdrinker I would like to try GrapheneOS for the first time, but after I read the manual installation website I'm not sure I can do it. Because the website mentioned "dl.google.com" multiple times and I am actively blocking all of Google-related domains and IPs on Network & Firewall level. I don't want to make any exception towards Google just to install this GrapheneOS.
Those instructions include downloading the Android command-line tools, which are written by Google, hence downloaded from them. Various Linux distributions contain those tools in packages, but those are typically very out of date, so downloading directly from Google is the reliable way to do a command-line installation. Downloading via curl does not involve cookie-based tracking unless you manually enable it (source).
rumdrinker I guess WebUSB-thingy uses dl.google.com too - and if not, how will it work?
I suspect the WebUSB installer does not download the command-line tools, because it can't run them. I suspect the WebUSB installer does not contact Google, but you can give it a try.
rumdrinker Will it works without internet?
No, because it downloads the GrapheneOS installation image from the GrapheneOS web site.