you could only overlay it as a subtitle file, bit of work to do that, unless you're talking about a major speed difference that blind Freddy would notice because it was so far off then you're unlikely to get anywhere if you want to take it to court, if the speed difference was that obvious the video would demonstrate that without any speed info being there, if you're talking about 10mph or so difference they're unlikely to accept any evidence you produce based on the camera GPS as it's not a calibrated device
adding the info after the fact probably won't help you either as the evidence is then tampered with, even if you're not trying to cheat in any way it's still not the original video once you go adding anything in so they would considering that as being tampered with
not trying to rain on your parade, just looking at it logically