Fit a throttle to a pedal assist.bike? Can I fit a throttle to a pedal assist Viking Gravity ebike?
If yes what is needed other than a throttle?
Thanks
It's going to depend on several things:

The controller
The type of pedal-assist detection
How good you are with electronics
Whether you can get a full kit for that model

If you can't get a suitable kit, you either need to be good at electronics (design and assembly), or pay someone who is.
I've sketched out a design in the past (on paper - I don't have them any more).  Here it is, with some other options.

One plausible and not too invasive idea involves faking the input to the pedal sensor with a throttle that gives pulses at a suitable rate.  This is what a former colleague was going to do after discussions, but analysing and generating electronic signals is the sort of thing we both do for a living.
Replacing all the controls and motor drive circuit may be an option, but how could you be sure the new bits would fit and be compatible with the motor and battery? Is the charging controller integrated into the same module?
The existing control circuit may have an unused throttle input (throttles are popular where they're allowed so it's not unlikely.  It may be very easy to interface or very tricky, depending on how much is present.