@John Fitzgerald hi, normally, participants will see graybox, when the presenter select application level presenting. if there is any application open at top of shared application at the presenters computer, participants will see the gray box, because they only allowed to see the shared application. you can try below options.
1. share complete screen (not application level) which will show everything on your screen to participants.
2. connect second monitor and share second screen to participants and use 1st screen for other applications simultaneously.
Please rate this and mark as solution/answer, if this resolved your issue
Good luck
KB