Filling "blank" space with a 0 Altitude value inside QGIS

Hello,

When importing a GeoTIFF into Gaea, the software tries to create a rectangular image (hope I’m explaining this clearly) and applies a very large negative value (e.g., -3200 m).

Is it possible to fill that space in QGIS with a value of 0 and thus avoid this problem?

Thanks!