This is testet under Gnome-shell [3.22.3] with:

BlueZ : 5.47-1

gnome-bluetooth: 3.20.1

pulseaudio-bluetooth/pulseaudio: 11.1-1

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Preference: you choose bluetooth and Gnome while installing Antergos

*Unpair your headphone first. (with Gnome-Settings or bluetoothctl)

To make your headphone auto connect each time you enable it, we need to enable the switch-on-connect module from Pulseaudio:

sudo nano /etc/pulse/default.pa

 put this two lines to the end:

# automatically switch to newly-connected devices 
load-module module-switch-on-connect

Do you use gdm?

then do this:
Prevent systemd from starting Pulseaudio anyway with socket activation :

This two commands will do the trick:

sudo -ugdm mkdir -p /var/lib/gdm/.config/systemd/user sudo -ugdm ln -s /dev/null /var/lib/gdm/.config/systemd/user/pulseaudio.socket

Today a find also a package to solve this GDM issue: 

pulseaudio-bluetooth-a2dp-gdm-fix [yaourt -S pulseaudio-bluetooth-a2dp-gdm-fix]

Continue for everyone:

Now it is time to connect and trust your headphone:

bluetoothctl is the tool we use here from terminal:

bluetoothctl
power on
agent on
default-agent
scan on
pair XX:XX:XX:XX:XX:XX < your headphones "ID"
trust XX:XX:XX:XX:XX:XX
connect XX:XX:XX:XX:XX:XX
scan off
exit

Check if your headphone is listen under sound devices and is connected via A2DP profile:

May you get only Headset Head Unit (HSP/HFP) this is for what it say Headset for telephone alike use with Mic, for HIFI Headphone choose High Fidelity Playback A2DP.

As far as i can see it will take the last state for each new automatic connection.

And it is done!

To see if it works > reboot system, if you do it right and for shure you are lucky to have choosen good hardware ,or the right one working together 😉

Now, each time you power on your headphone, system auto connecting it with High Definition Audoprofile [A2DP].