Dear Alex: I don't know what the dice game Caesar referred to,
but evidently there must have been a game in which only one die was used. I have read that animal knuckle bones could be used
The game could have been as simple as throwing one bone, then
throwing a second one, trying to match the position of the first one, and losing your wager if you failed to achieve a match.
If carefully made numbered cubes like modern dice had been u;sed, the archaeologists ought to have found thousands of them. I'll go see if I find anything about that.

I found several sites indicating that cubic dice go back three thousand years or more. Here is a site with pictures of ancient dice. However, no clue as to rules of games in Caesar's day.
http://www.geocities.com/ladysveva/games/DiceImages