Within Australia, CAO 48 supplies the legislative requirements with respect to a single day off, which are very similar to your description of a 'G' day.
Next, many airlines have an exemption to CAO 48, with various amendments to the requirements found in CAO 48, in relation to flight and duty limits, as well as time free of duty.
Additionally, often your contractual terms will specify greater time free of duty requirements surrounding single and multiple days off. I don't believe there is any scope for contractual requirements to breach the CAO nor exemption. This would normally be found within your EBA, or possibly your airline's operations manual suite.
Possibly some operators are under an FRMS though I'm not entirely sure. Nor am I familiar with the requirements surrounding an FRMS.
I hope that helps.