Yes, garden centre is a noun.
noun
No it is a place - a noun or "to centre" is a verb
centre
Whether a word is a noun or a verb depends on its function in the sentence. Centre is a noun in the following sentence: People thought that the Earth was the centre of the universe. Centre is a verb in the next sentence: "All these theories centre on two battles in Estonia" (quoted from Wikipedia)
Common noun
The noun form of "central" is typically "center."
common
Pea is a common noun, and peas is the plural...still a common noun.
A common noun.
Most definitely a common noun.
Camel is a common noun.