So I need to get to bed soon but before I go crash I thought I'd check the thread.
Become one with Gaea itself. See through the animals and feel through the trees. Basically be anywhere where life thrives.
Yeah this is basically never going to happen. To completely usurp Gaea you would have to have Perfect level DM in every single concept that exists. And if you mean just controlling all the non-sentient creatures and all the plants on Gaea, while that would in fact be possible with Perfect level DM in Nature(barring any creatures specifically tied to other Deities), by the time you become that powerful there will be Gods with Nature DMs to oppose you.
More practical would be controlling all the life in a specific territory, concentrating your power in one area and not spreading yourself out. That would help you fight off the influence of other divine beings. If you designed some sort of power booster that enhances your influence over an area you could even control a pretty large area this way; though someone destroying your boosters would weaken your control in that territory.
Alternatively you could kill all rival Gods with any power over Nature, but that kind of behavior tends to piss people off so it is pretty risky.
But with all of that said, this is all theoretical because that kind of control requires Perfect level DM. Creating a creature specifically designed so that you can remotely control them is within your abilities however.