A number of high-profile European clubs are reportedly keen on his services, with the likes of Paris St Germain, Juventus and Atletico Madrid among those said to have shown an interest.
The 22-year-old Brazilian has impressed in the Bundesliga so far this season, scoring 11 times in 21 appearances as his side occupy 10th in the table.
Firmino has another year left on his current deal at the Rhein-Neckar-Arena and Gisdol was quick to dismiss suggestions that his star man could be set to leave.
"These are just rumours," he told Sport-BILD weekly.
"Roberto Firmino is under contract at our club and he is a first-team regular with us.
"In addition, everyone should be aware why Roberto is playing such a good game at this point in time."