How to findout the directivity from gain? & what is the unit of gain & directivity

Antenna gain is measured relative a defined reference. A common reference is an isotropic antenna, and dBi is then a common unit. Gain can also be measured as result of before and after a modification of an antenna => unit is: dB[relative old antenna].
Directivity is related to the antenna average radiation pattern. Compare front-back ratio for a Yagi-Uda antenna. It is a unitless ratio. For calculating logarithmic ratio is dB common as a unitless numeric base.
Gain dBi can be very low even if directivity dB is high.

Directivity is usually expressed in db, with isotropic it would be dbi.
and same goes for Gain.
They are related in a way that, "Directivity defines that a particular antenna has maximum gain in this direction as compare to all other directions of radiations".

Power gain (or simply gain) is a unitless measure that combines an antenna's efficiency whereas Directive gain or directivity is a different measure which does not take an antenna's electrical efficiency into account.

Buzzelectrician

Antenna gain is the mulptiplication of directivity and its electrical efficiency factor.

Gain accounts for efficiency. Directivity doesn't.

Gain = Efficiency x Directivity.

there is a kind of standard antenna，just use a field intensity indicator testing standard antenna and tested antenna，then you can get tested antenna' gain.

For making measurements you can refer to [ http://rcaverly.ee.vill.edu/crcd/ant...re/measure.pdf ]. The most common methods are using either the two antenna gain method or the three antenna gain method.

You can use two standard antennas to characterize your range, and then replace the receiving antenna with your test antenna. This is also a common method.