"HGF" or hollow gold filled jewelry is considered a fine jewelry and should be treated with the same caution when cleaning.
To clean HGF jewelry a solution of Mr. clean, water, and ammonia can be used. Mix half Mr. Clean and water with a small amount of ammonia. This is safe for all types of fine jewelry and is used by many jewelry stores. Allow the jewelry piece to soak for 15 seconds in this solution then rinse with water and dry.
It could mean 18 karat "hollow gold filled". I'm not familiar with a manufacturer's hallmark that is "HGF".
I haven't seen vinegar clean jewelry but I saw my girlfriend clean my tarnished earrings with toothpaste. I was amazed.
Clean your jewelry with isopropyl alcohol, or at least 80% cleaning alcohol if you have that.
Mercuryfluoride
HgF
A solution of half Mr. Clean and water with a small amount of ammonia can be used to clean gold jewelry. This same solution is used in the ultrasonics of many jewelry stores including chain jewelry stores. I will add a link to a full article about cleaning fine jewelry.
Molecular formula is Hg2F2 . The empirical formula is HgF . For empirical formulas you reduce the numbers as far as possible.
18k hgf would stand for 18k heavy gold filled ( not solid gold )
The Da Vinci Jewelry I know of is considered fine jewelry and since fine jewelry needs special care I will explain how to clean fine jewelry which will be good for all types of jewelry. A solution of half Mr. Clean and water with a small amount of ammonia can be used to clean all types of fine jewelry. Soak the jewelry piece for 15 seconds then rinse in cold water and dry. This solution is used by many jewelry stores I know of and they use it for all types of jewelry including inexpensive jewelry, fine jewelry, and even opals, and emeralds. A bowl can be used, but an ultrasonic is better. Repeat if the piece is extra dirty.
You have to remove the jewelry and clean it, including the piercing.
Always make sure to clean both your jewelry and the piercings themselves with hydrogen peroxide to avoid infection. Store your jewelry in a safe and clean location. Never purchase plated jewelry.
It depends on what the jewelry is made of.