Ghain and Ghaim are two synonyms. They signify the spreading of the clouds. It refers in this Hadith to the fact that the Prophet (PBUH) was reciting the Names of Allah and remembering Him all the time. But sometimes due to being too busy in the affairs of the public or due to human needs, there was a pause in this eternal remembrance. Although such a pause was only momentary, this momentary pause was very much disliked by him; and considering it a slackness on his part, he prayed for forgiveness from Allah. In this there is an important lesson for us.
In spite of our many sins and negligence on our part, we do not turn to Allah and seek forgiveness; while, although our Prophet (PBUH) had all his past and future sins forgiven by Allah, he sought forgiveness very often.