If the gasket of your watch needs to be replaced you may change it to an O-ring with dimensions 32 x 0.6mm.
Please have the replacement performed by a professional jeweler.
https://support.withings.com/hc/en-us/sections/4411845138449-Move-ECG
https://support.withings.com/hc/en-us/sections/360003273634-Maintenance