VOICED VELAR STOP [g]
VOICED UVULAR FRICATIVE [ʁ]
Gimal with dagesh is pronounced like English "g". Gimal with rafe, which typically occurs after vowels, is pronounced almost as if you were "gurgling" low in the back of the throat, but with a smoother consistent flow of air so you don't get the "gurgling" rumble sound. For those familiar with Arabic, it is pronounced like the letter ghayin (غ). It is important to keep this sound distinct from Tiberain resh, which is pronounced in the same place but with a "gurgling" sound.
SOURCES AND FURTHER READING
Khan, Geoffrey. 2020. The Tiberian Pronunciation Tradition of Biblical Hebrew. Cambridge: Cambridge and Open Book Publishers. §I.1.3.