The 20-year-old impressed at the 2022 World Cup in Qatar, where he played every minute of Croatia's tournament.
Gvardiol capped off his World Cup by scoring in the third-place play-off victory against Morocco.
Croatia's deep run at the tournament created many new fans for Gvardiol, but Leipzig remain determined to fend off interested parties until at least 2024.
"I'm in contact with his agency," RB Leipzig sporting director Max Eberl told reporters today.
"We are in very open talks. Our position is clear: Not this winter under any circumstances, and it's also not our intention to let him go in the summer of 2024.
"Beyond that, it's hard to say. But we are definitely planning with him for the next year and a half."
Gvardiol still has three-and-a-half years left on his current Leipzig contract.