All Amazfit Bips when powered up for the first time demand that you connect in via the official App. This is not necessary, you can get started with Gadgetbridge only. (This might be different from firmwave version 1.1.6, read section Pairing below).
Steps to follow:
If you had your watch previously paired to your phone’s bluetooth, make sure to unpair it first.
When starting Gadgetbridge the first time, it will automatically attempt to discover and pair your Amazfit Bip. When your Bip notifies the pairing process, tap it quickly a few times in a row to confirm the pairing.
New devices and older devices with updated firmware require server based pairing. This means, that you absolutely must use MiFit app to make the initial pairing, retrieve the pairing key and then use this key to pair with Gadgetbridge. Currently, Mi Band 4 and Amzfit Bip Lite are known to be the first device with this enabled from the beggining. Read page Server based pairing for further details. There might be other devices, like the Amazfit Bip, following, but this is not confirmed yet.