It’s actually a bit simpler to show the list of all staff but “flag” those who’ve completed 6 or more hours - i.e. make those rows appear visually different. IOW, the list wouldn’t be filtered per se, but the individuals who satisfy the condition will “stand out” in the list. This approach would make use of a formula in a Conditional of an element in the RG.
That said, it is indeed also possible to actually filter the list. That approach would implement the formula as part of a filter after the search query.
The following thread has an example which illustrates both approaches, but that example is actually more complicated than you need, since it uses multiple conditions, and you have only one (6 or more hours of training completed).
Perhaps it will help though…