Basic Question: Dipole gain dBd or dBi ??

A basic question!
I have two Dipole antennas, which are identical and I conduct an experiment. I use VNA to connect them to two ports and measure S21. How do I calculate the gain of using Friis transmission equation?
Using ,
S21 = Path Loss + Gt + Gr
Here Gt = Gr = G
So
G= (S21- Path Loss)/2

and knowing measured S21 and calculated path loss , I can find out Gain of antennas. But will this be of a unit dBd or dBi? :shock:
Lets assume , I get S21 = -25dB and PL= -35dB
therefore putting values, Gt = Gr = 5dBi or 5 dBd??

I guess it can't be dBd because for a dipole a gain is 0dBd but these antennas is not isotropic either. :???:

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I understand 0dBd = 2.15dBi.
Does that mean, in every measurement I should get dipole antenna only 2.15dBi? I am working on frequency 1.5GHz.

The path loss formulas take into account the gain with respect to an isotropic radiator, then you have to use dBi.