i need to insert the g values for earthquake in caesar.

according to UBC 1997 (2of3), section 1632.2, i calculated Fp.

now: how do i convert Fp to g?

example Fp=0.36 --> g= ???

you have calculated that Fp= 0.36 * Wp.

Then g = Fp / Wp = 0.36

Wp = the weight of an element or component. (<--written in UBC97)

when i have g= Fp / Wp am i considering the unit weight (Wp=1), so g is referred to a unit weight?

(note: Wp should be the mass, not the weight?)

A little bit more complicated.
Weight is a force, Fp is a force, so you determine that horizontal Fp is a fraction of Wp.
In other words you determine that the horizontal acceleration is 0.36 gravitational acceleration or 0.36g.

Some confusion is created because in United States customary units, the pound can be either a unit of force or a unit of mass.

The other thing is that in fact we determine an horizontal acceleration u= 0.36g, we do not calculate the g which is the gravitational acceleration.
We still ask how much is g when we should ask "what fraction of g" should we use.
When we insert in Caesar the 0.36 we specify that Caesar need to calculate a horizontal force equal 0.36*weight
do not know if I succeded to clarify.

where can i find the formula "Fp= 0.36 * Wp" in UBC1997?

the value of 0.36 was your example

the formula is in 1632.2

Fp = 4.0 * Ca * Ip * Wp (32-1)

or

Fp = ap*Ca*Ip/Rp*(1+3*hx/hr)*Wp (32-2)

replace with your applicable values and you obtain the horizontal acceleration (let's call "u")

Fp=u*Wp or u=Fp/Wp

so u=4.0 * Ca * Ip or u=ap*Ca*Ip/Rp*(1+3*hx/hr)

I think that for those familiar with physics following International System of Units, it's more easy to consider (with "u" as per Dan notation)

Wp=g*mp where g is the vertical acceleration

Fp=u*Wp=u*g*mp=(u*g)*mp where u*g is the horizontal acceleration by seismic event

Because input of horizontal acceleration is expected to be in fractions of "g", input must be "u".