The Robins finished as runners-up in League One last season, behind champions Scunthorpe, and are flying high in the Championship this term.
"I won't discuss our targets, but Dele is someone we know about. There has been an enquiry," Johnson told the Western Daily Press.
"There are rumours of people we are interested in, rumours of people we are not interested in and rumours of people we have never even heard of.
"People will just have to keep their eyes on the newspapers and wait to see what happens."
Adebola would provide competition for Lee Trundle and Darren Byfield, with forward Enoch Showumni rumoured to be leaving Ashton Gate during the transfer window.