Been to Crete many times but never done any cycling.
There are hundreds if not thousands of little tracks (single and jeep) leading off main roads all over Crete.
Not sure what access or privacy rules affect peoples driveways/paths but I'm sure the Cretans won't shoot you, they're a pretty friendly lot, although most own guns!!!
Suggest you head inland (south) from Gouve for the hills. Once you're off the tourist strip around the coast, it should be MTB heaven……just take plenty of water and sun cream with you.
A scouting trip in a car or on the bus may pay dividends. West of Heraklion is quieter and maybe more scenic.