Gray Whales are a specific type of whale and are unique in that they have the largest migration pattern of any mammal. These mammals are capable of traveling "10,000-12,000 miles round trip" every year.
Typically, gray whales spend summers far north in the waters off of the coast of Alaska, in the Bering straight and Chukchi sea. As winter comes around, the whales migrate from these northern waters down the Baja coast of southern California and into the waters off of the shores of northern Mexico.
The reason for this migration to warmer waters is two fold. Firstly, as northern waters become colder food becomes more scarce. Moving south to warmer waters provides greater sources of food in the winter. Secondly, it is in these warmer southern waters that gray whales mate and give birth to their young. The gestation period for female gray whales is 12 months, meaning that in one migration season a female will become impregnated, returning the next migration season to give birth. Hope this helps!