Hello,
If you don't know the Pt*Gt product, make yourself a dipole, and check the VSWR to make sure that matching is OK. In that case Gi = 2.1 dBi. You can also use another antenna, but you should be confident about the gain (standard gain horn?).
The power received by an antenna: PRX = PFD*Ae, PFD = power flux density in W/m^2. PRX is received power in W.
Ae = effective aperture of antenna = Gi*lambda^2/(4*pi). Gi not in dBs
E field PFD relation: E = sqrt(PFD*377) in V/m
So based on the power reading from, for example, a spectrum analyser, you can determine the E-field at your target.
Make sure you check for uniformity of the field close to the target as too strong reflections will make your results useless. Also don't forget to correct for cable loss as this can be significant at 2.45 GHz.