data sheet only contain capacitance at specific junction voltage and specified frequency, and it changes with respect to voltage and frequency
You are misunderstanding the datasheet, I think. Usually capacitances are specified with a measuring frequency (1 MHz)
and a voltage. Additionally, most datasheets also have a diagram of the voltage depency. If not present, you can assume the
voltage depency to be similar to other transistors of same technology, that have this data.
But what does the 1 MHz mean? In my opinion, it's simply a sufficient low frequency to observe a pure capacitance.
The capacitance is almost frequency independant, but at higher frequencies, it doesn't appear as a pure capacitance, you have to use
e.g. S-parameters to describe the transistor impedances. At 1 GHz, also package impedances, particularly inductances play
an important role, they are also considered with S-parameters. Either you have S-parameter tables from the manufacturer, or
you can try a SPICE simulation, but it's only meaningful with a correct package model.