Wouldn't it be easier to define the speed of light in vacuum as 1?
Thus you would have measurements of speed in the milliC (~300km/s) and microC (~300m/s) and nanoC (0.3 m/s). This has the advantage of being abstract to both time and distance.
An external measure of time is also fairly easy to standardise. The half life of tritium would be a fairly good one. The half life of any common decaying element can be used.
With time and velocity codified, distance just falls out of the equation, as does acceleration both being derived between distance and time.
There are problems carrying specific planetary units between systems. G, day, month, year are all such units, and are dependant on specific metrics that cannot be easily carried from the originating planet. None of these units make it as far as another planet in this system, let alone to interstellar space. Even the old definition of a metre (10^-7 of the distance from the equator to the pole) is based on terrestrial measurement.
From a gaming point of view you want to use terestrial units though. The units used by your characters are not going to be talking metres hours and days in the OTI, I have always assumed that they used Vilani based units, or some codified external measurement. I use earth based measurements so that my players will not be as confused.
For G you already have a non-terrestrial measurement, the newton.
Tangentally, I found an interesting site talking about what happens when you do travel faster then light. It creates something called
Cerenkov
radiation. Interesting stuff.