Błędnie: He's said to speculate about gold- is that true?
Poprawnie: He's said to speculate in gold- is that true?
"Spekulować (czymś)" to 'speculate in (something)'; 'speculate about / on (something)' oznacza "spekulować o (czymś)"- czyli rozważać, domyślać się, zastanawiać.