SALEM – A Lynn man who admitted earlier this month that he sold heroin to an undercover police officer during a city sweep of street level dealers last fall was sentenced to 2 1/2 years in the House of Corrections.Brian K. Gibson, 44, of 21 Whiting St., changed his plea to guilty last week to a charge of distribution of heroin before Superior Court Judge John T. Lu.Sentencing had been delayed at the request of the defense.In the meantime, Gibson remained free on his own personal promise to return to court for the imposition of his punishment.In asking the judge to adopt the sentence agreement, defense lawyer Christopher Norris told Lu that it was an “appropriate and fair” disposition of the case.Assistant District Attorney Kim M. Faitella said the crime occurred on the morning of Sept. 29 when Gibson was approached by an undercover police officer at the corner of Lloyd and Washington streets and asked Gibson if he could get him a bag of heroin.Gibson made a phone call, left and came back with one bag of heroin, which he turned over to the undercover officer for $60 in marked bills. Police moved in and arrested Gibson. Marked bills were found on his person.Because it is a House of Corrections sentence instead of a state prison term, Gibson will serve about 15 months behind bars before he is eligible for parole.