If I choose an implant size that's a cm below my BWD will I end up with a gap? (photos)

First of all take all that you have learned about bwd and throw it away. It is not a good way to judge implant size. I think the sizes you mention are far too large but if you want to look like the photo you sent, it's your decision. The sheer weight of them over the next twenty or thirty years will really cause problems

While your question is a good one, an evaluation in person by a board certified plastic surgeon is necessary to properly determine which implant would be best for you to achieve your desired outcome. It appears that you already have a scar below your breast which suggests you have had prior surgery. The implants you suggested are very large and for the purposes of augmentation (not reconstruction) are highly ill advised. You are likely to encounter significant complications early or late with such large implants, and almost certainly require additional surgery in the future to repair the complications.

I recommend you select a board certified plastic surgeon whose results align with your own sense of aesthetics, and I suggest you seek a pretty and proportionate goal for yourself. Best of luck to you.