@ Ritesh,
Please always provide full birth info.
It always depends for which reason you wish to wear the gemstone, it depends on your horoscope too. At times it is required to force Rahu onto Surya if Surya is troubling and you need to use Rahu to your benefit.
The ring finger is for Surya, Gomed is for Rahu. So if your astrologer has suggested you to wear Rahu on ring finger then either he is not an astrologer or an intelligent astrologer.
Regular astrologers suggest wearing Gomed (Rahu) on the middle finger (longest finger).
If you know that the astrologer who gave you these advise is really intelligent then go for it, or else follow the regular rule (middle finger).
Wishing you good luck, May God bless you and over all!
CHEERS!!