Some nouns that are ideas (abstract nouns) that start with G are:
i have no idea but i think it starts with a g!
Georgia, Greg, Garth, Germany
germ
silliness, sound, skepticism, soulfulness, softness, surrender, simplicity, etc.
An idea that is a noun: idea
"pyramid" is a 7 letter common singular noun that clearly begins with "p" a noun is a person, place, thing, or idea- a "pyramid" is a thing
An "island" is a common noun that starts with the letter "i."
Guyana starts with a "G"
garden soil starts with G
Hopeful is an adjective, not a noun. The noun form, hopefulness is an abstract noun (if that's what an 'idea noun' means).
Wikipédia is a French noun that starts with w.
The plural of idea is ideas.