Definitely follow these instructions on configuring udev on your system: https://wiki.cyanogenmod.org/w/UDEV

On a Gentoo system also ensure you have emerged: dev-util/android-tools

Even if you do all the steps and still do not get an Authorisation Request when plugging your phone over USB and your adb devices is empty, then you probably have to do the following in addition to everything mentioned already:

Plug your phone over USB and get a list of the attached USB devices by executing as root: 

# lsusb
Bus 001 Device 055: ID 0e8d:201d MediaTek Inc. 
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
...
..

The 4-character string highlighted with red is the vendor id of the mobile device. Of course if you have a different phone this will vary.
Now edit, or create if it doesn’t exist the file ~/.android/adb_usb.ini and place on a single line the prefix 0x followed by your vendor id:

# ANDROID 3RD PARTY USB VENDOR ID LIST -- DO NOT EDIT.
# USE 'android update adb' TO GENERATE.
# 1 USB VENDOR ID PER LINE.
0x0e8d

Then restart the adbd:

adb kill-server
adb start-server;
adb devices

If all other steps were done correctly, you should see on your phone an Authorisation Request. Go ahead and accept to be able to access your phone from your linux host.

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