those initials could be of manufacturer, or person who
owned the ring, is this a new ring? If so where purchased should be able to tell you...
HGL could be the symbol or the initials of the manufacturer of the ring. Nowadays, it is quite oftenly seen that jewelry manufacturer's engrave the symbol of there company or initials of some sort in order to create brand awareness.
Other people asked questions on similar topics, check out the answers they received:
Other people asked questions on various topics, and are still waiting for answer. Would be great if you can take a sec and answer them