A former Fredericksburg college student who served 17 months in a Virginia prison in a 1980 drug case has been resentenced to 10 years' probation after defense lawyers argued that Virginia State Police withheld evidence in the case.

Bristol, Va., Circuit Court Judge Charles B. Flannagan II on Wednesday reduced the 20-year prison sentence of the former student, Robert W. (Rusty) Gibson, to 15 years and ordered that Gibson remain free on probation.

"I'm really grateful to the judge for the hearing." Gibson, 28, said yesterday. "I'm going to try to pick up my life again."

Flannagan had ordered Gibson freed in July pending Wednesday's resentencing hearing.

Gibson's lawyers, John K. Zwerling and Michael S. Lieberman of Alexandria, alleged that state officials failed to disclose at Gibson's trial that an informer in the case was a fugitive from justice and a second informer was a habitual criminal who was being shielded from prosecution by a state police officer.

Gibson contended that one of the informers had pressured him into involvement in the crime--the $28,000 sale of a pound of cocaine to a state police undercover officer in a Bristol motel room.

Lieberman said yesterday he did not dispute that Gibson, then a senior at East Tennessee State University near Bristol, had been involved in the sale. He contended, however, that Gibson's role as an intermediary was minor compared to the part played by the informers, who were protected.

The undercover officer testified at Gibson's trial that he did not know the informers' identities or whereabouts, when, in fact, he was in daily contact with one and personally had arrested the other on a felony charge a year earlier, according to court papers filed by Gibson's attorneys.

Gibson was convicted of possession of cocaine with intent to distribute. He originally was sentenced to 30 years in prison, but the term was later reduced to 20 years.

Gibson said he may return to school.

"I need to make some money so I can get back on my feet," said Gibson, an electrician, who broke his right foot two weeks ago.