It depends on the exact meaning; there is not a one-to-one mapping between Glück and either fortune or luck.
Luck is related to chance, and would be the obvious choice in a gambling context: Glück im Spiel -- "luck in gambling". Or in the case of having avoided an accident: Glück gehabt -- "I was lucky."
Fortune relates more to circumstances which are less random: I was fortunate to have had support from my peers, which in German might also be translated as glücklich. Its nominal use is mainly in the 'career' sense, as in make your fortune, perhaps translatable as sein Glück finden. This latter example could also be find happiness. The difference here is that happiness is not necessarily linked with success, which fortune would suggest.
The use of make in relation to fortune also indicates that it is something you are mostly responsible for yourself; you can also find your fortune, but that is rarer and usually then linked to (random) discoveries. Glück works with both machen and finden.
To summarise: both are possible (plus happiness), luck seems more likely, but ultimately it depends on the context in which it was used.