One gray is exactly 100 rad. Both measure absorbed dose — energy deposited per unit mass — and they must not be confused with the sievert, which measures something different despite having the same dimensions.
About the units
The gray is the SI unit of absorbed dose, defined as one joule of ionising radiation energy absorbed per kilogram of matter. The rad is the older CGS-derived unit, defined as 100 erg per gram, which works out to 0.01 Gy. Both are purely physical: they say how much energy the radiation deposited and nothing about what it does to tissue. That distinction is the important one. The sievert, which uses the same joules per kilogram, multiplies the absorbed dose by weighting factors for the type of radiation and the tissue affected, and it estimates biological harm rather than energy. One gray of alpha radiation and one gray of gamma radiation deposit the same energy and cause very different damage.
The exact factor
The factor is exactly 100, since a rad is 0.01 Gy by definition. Reference values give the scale: a chest X-ray delivers about 0.1 mGy to the lungs; a CT scan of the abdomen 10 to 20 mGy per organ; radiotherapy delivers 50 to 70 Gy to a tumour in fractions over several weeks; and a whole-body dose above about 5 Gy is usually fatal without intensive treatment. The enormous span between diagnostic and therapeutic doses is why the prefixed forms matter as much as the unit.
Where you meet this conversion
Radiotherapy above all, where treatment plans are prescribed in gray and the total is divided into daily fractions. Radiation processing of food and medical products, materials testing and dosimetry of electronics for space use all specify in gray. The rad survives in older American literature and in some legacy equipment, so historical dose records need conversion. Anyone reading a radiotherapy protocol from before the 1980s meets rad throughout, and treating a figure in rad as gray overstates the dose a hundredfold.