I think this isn't an equipment issue. A prong should be fine. teach a focus command indoors and work up to outdoors using the ball or food as a reward. Ask for super short commands for now.
ex: if your dog can focus for 2 minutes inside before a reward, i'd reward after like 20 seconds outside and work up. 6 months is young. Mine didn't start getting more focused til later
i gave up on the gentle leader. Hated that. I tossed it in a box the time my dog saw me pull it out and hid under the kitchen table. not worth it to make my dog miserable