Answer:
By referring to the driveway as "gravelled" rather than as a "gravel driveway", it means that the driveway was man-made, filled with gravel, and not originally made of gravel.
Explanation:
To first understand the use of "gravelled" instead of "gravel", we should refer to the two words separately.
First, "gravel" is a noun, referring to the fragments of rocks, loose pieces of rocks. On the other hand, "gravelled/graveled" is a verb referring to the act of covering an area with gravel.
Now, looking at the use of the word "gravelled" in the novel, the narrator describes the driveway of Judge Miller's Santa Clara Valley house. And in mentioning that the driveway was "gravelled", it means that the driveway was filled with gravel, a manmade driveway, covered by gravel.
