You could do it. As an employee I believe I would find it somewhere between cheezy and annoying/aggravating. Gamification for sites like the stackexchange family primarily works (in my mind) because of the voluntary nature of the interaction. Adding it to a mandatory process would tend to feel outright manipulative.
If you are referring to mandatory training, like health-and-safety, then just good old managerial top-down dictates should suffice. If all employees need first-aid training, for example, you sign them up, and if it is part of their employment contract, you fire them if they do not fulfill that requirement.
You seem to be talking about the sharpen-your-saw type of training. Training that would allow them to do their job better, perhaps, if they took it? From that perspective, if the training itself is not enough of a reward, how is getting a little virtual gold star going to make it more palatable and not condesending?
If I believe I know everything I have to, in order to get my job done, then little prizes are not going to change my mind. What is the real benefit to me for taking this training? Do I get more pay? A promotion? If I make more widgets per shift, who benefits?
Once people make the mental link to their own interests, the need for gamification would go away.