All subtypes of the creatures are removed by the subtype "Forest". All abilities are removed and replaced with "Tap: add G to your mana pool" and the creatures cease to be creatures, since you do not state the opposite.
No, actually it wouldn't.
1.) Firstly, unless it was actually a land, it wouldn't have a mana producing ability (rule 212.6d). So, while it might get the "Forest" land (sub)type, unless it is actually a land, it doesn't gain the mana ability.
2.) Secondly, you are only replacing a subtype here, not a type. You can change the subtype of an object without changing the type of the object. It's only when you change the type of an object that it would lose the current type
and subtypes. But changing the subtype doesn't affect the type at all. That's basically what 212.1b gets at.
So, with the first wording, they would indeed be "Creature - Forests", but they wouldn't do anything special. You have to wipe out the existing type (change "Creature" to "Land") in order for it to work.