Are the following workers — a beggar, a thief, a smuggler, a gambler? Why?

No, a beggar, a thief, a smuggler, a gambler cannot be called workers. A worker is involved in a production activity that contributes to the GDP of a country. As none of them (a beggar, a thief, a smuggler and a gambler are involved in any legal economic production activity that contributes to the national income of the country, hence, none of them can be regarded as workers.