To install GrapheneOS on my Pixel 6a using Linux Mint 20 in the first place I did:
sudo apt-get update,
sudo apt-get install android-sdk-platform-tools-common and
sudo apt install adb. After that I could just follow the original instruction [https://grapheneos.org/install/web] using Chromium.
Actually I was looking for how to transfer files between my Pixel 6a and my Linux Mint 20 laptop when I came here. Since I'm a newbie and I found it hard to find, I like to share it here.
First of all I followed this video [https://www.youtube.com/watch?app=desktop&v=1EKQbKHSBb8]. Second of all, you have to enable "File Transfer" under "USB Preferences" in GrapheneOS. Using the original USB-C cable also helps. After those 3 things, I was able to see the phone in the Nautilus filebrowser and able to copy files. Now you can easily copy photos to your laptop or put music on your phone.