GAP configuration - advertising TX_POWER

Hi !

I'm working on a project and I've got some questions about GAP configuration :

Here is the organisation of my system :

Phone 1 Phone 2
Device 1

Phone 1 is bonded with Device1. But want Phone2 to get some information about Device1. I've seens that it's possible to advertise while being bonded.
I want to insert the Device1_ID, but also the Tx_power in the advertising packet. Is that possible ? (I think so).
Actually I want phone 2 to be able to estimate the distance between it and Device1.

For that I've understood that I can configure the GAP layer with the tag GAP_AD_TYPE_TRANSMIT_POWER (gap.h). But I can't find where.
Or is it already implemented ?

Thanks in advance for your reply !