My understanding (possibly incorrect) is that cardio reduces bg, and weights will increase it. Having said that, different people's bodies vary quite a lot in how they react to exercise so it's a question of trying it out and seeing. When I go to the gym I rarely find it changes my bg much so I
1) blood test before I start (and have something if it's less than 5.5ish)
2) carry glucose and my meter and test if I feel shaky. (Of course, it can be hard to distinguish between a hypo and "I want to die now, please may I stop exercising?" but I've yet to keel over at the gym, though I do sometimes go hypo and take some sugar.)
When I was using the freestyle libre (I've stopped now because I became allergic to the sensors) it was particularly awesome at the gym, as I could do a quick swipe of my arm in between the different exercises.
Disclaimer: I just go for an hour for cardio and core type exercises.... (5 years ago congenital weak back issues were making it uncomfortable to stand/walk for more than 30 minutes but long term exercising has meant I can go back to long walks with my dogs and husband without any hint of discomfort
)