G thread: 55 degree non-threaded sealing pipe thread
G is a 55 degree non-threaded sealing tube thread, is a Wyeth thread family. Marked as G, means the cylindrical thread. GB / T7307-2001.
The difference between metric threads and inch threads:
Metric threads are represented by pitch, while US-inch threads are represented by the number of threads per inch;
Metric thread is equilateral 60 degrees tooth type, inch thread is isosceles 55 degrees tooth type, American thread for the waist 60 degrees tooth type.
Metric threads in metric units (e.g. mm), American and British-made threads in inch units (e.g. inch)
If there is other information I have not mentioned, or if there is any mistake, keep me informed.