Nature:
* Garden
* Green
* Grass
* Grove
* Glitter (of sunlight)
People:
* Graceful
* Generous
* Gentle
* Good
* Grateful
Things:
* Gift
* Glimmer
* Glow
* Glass
* Goal
Abstract:
* Grateful
* Grace
* Glory
* Goodness
* Growth
Funny:
* Goofy
* Giggly
* Green (as in a "green thumb")
* Gargantuan
Ultimately, the best word for your "G" depends on the overall theme and tone of your acrostic poem. Consider what you want to emphasize and choose a word that fits the context!