cannot get A2DP mode work with my bluetooth headphones on Gnome

Same issue here, Ubuntu 15.10, Gnome Shell 3.18.2.

Unfortunately your workaround didn't work for me, I found the workaround/fix here that is based on the same arch wiki provided by you.

Here is what I did:

(1) run the following command in terminal:

sudo setfacl -m u:gdm:r /usr/bin/pulseaudio

(2) reboot Ubuntu or restart the pulseaudio by running the following command in terminal:

sudo pkill pulseaudio

Looks like no one reads wiki, so I had to dare and find the solution myself

1. First make sure you have installed pulseaudio-bluetooth

2. Add these lines to /etc/pulse/system.pa:

   load-module module-bluez5-device
   load-module module-bluez5-discover
   

3. Make sure you have these lines in /etc/pulse/default.pa (they should be already, btw)

   load-module module-bluetooth-discover
   load-module module-bluetooth-policy
   

4. Make sure you have trusted your bluetooth device, which you can do it as:

   $ bluetoothctl
   $ trust < mac address of your device >
   

5. Last most importantly for Gnome users:

When using GDM, another instance of PulseAudio is started, which "captures" your bluetooth device connection. This can be prevented by masking the pulseaudio socket for the GDM user by doing the following:

# mkdir -p ~gdm/.config/systemd/user
$ ln -s /dev/null ~gdm/.config/systemd/user/pulseaudio.socket

**Reference and for more: Arch Linux wiki on Bluetooth Headset

With Sony Headset mdr-zx550 pressing the button play/stop the option A2DP become available.