I had a dragon in the metal event and got the same glitch - event finished itself. I am pretty sure I got no gems.
Seems like there has been a lot of glitches as far as the Colosseum goes lately...especially with metal. Mine is finishing today so I guess I will see if I get a glitch when I try to finish it. I write down my gems and keep track of them so I will know if I get any or not but I have also noticed I had a couple gems show up with no notification of where they came from and it was before I did my next event in the Colosseum so not sure what happened there.