All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
According to the Hanafi Mathhab, it is not obligatory to wash impurity on the body before performing Ghusl, but it is only recommended.
Highlighting the Sunan of Ghusl, Al-Kanz (i.e. Kanz Ad-Daqaai'q, a Hanafi book) reads: “Among its Sunan is for a person to wash his hands, his private parts and any impurity that is on his body, and then he performs ablution etc… Besides, the intention is not a condition for removing impurity. Therefore, by pouring water over the body part that has become impure, one achieves purity and the state of impurity is removed.”
Therefore, you should know that it is not obligatory to wash the place a dog has licked before performing Ghusl, but if you do so, it is better and more appropriate.
Nonetheless, you should know that the impurity is not removed unless you wash it three times if it is invisible, as stipulated by the Hanafi jurists. Maraaqi al-Falaah reads: “The location of invisible impurity is purified by it obligatorily being washed three times.” [End of quote]
Also, the book entitled Fiqh Al-‘Ibaadaat (jurisprudence of acts of worship) of the Hanafi School, reads: “If the impurity is impurity of a dog, it should be washed seven times, and it is desirable for one of them to be with soil, as Abu Hurayrah said: "The Prophet, sallallaahu ‘alayhi wa sallam, said: “ To purify one of your receptacles which a dog licked inside, one should wash it seven times, the first of them with soil.” The Hadeeth means that it is recommended (not obligatory) as it was narrated that the Prophet, sallallaahu ‘alayhi wa sallam, was asked about a dog that drinks from a receptacle and he said: “Wash it three times or five times, or seven times." Also, Abu Hurayrah said: “If a dog licks something, wash it three times."" [End of quote]
Allaah Knows best.