FEDERAL PROSECUTORS, here and in Baltimore, were enraged by two of the sentences Judge Oliver Gasch imposed on contractors involved in the General Services Administration scandal. Judge Gasch, although less outspoken, was obviously just as angry at the public criticism the prosecutors directed at him. Unfortunately, their exchanges provide little evidence for making a sound judgment about who is right.

Judge Gasch has fined two contractors who paid brides to GSA officials $5,000 each and put them on probation for three years with the requirement that they do 200 hours of community service. In a document filed last Friday, U.S. Attorney Earl, J. Silbert charged that the sentences are so light they "give the appearance the crime does pay. They encourage dishonesty and corruption in government."

At first glance, that may seem right. According to Mr. Silbert's office, the government was cheated out of $1 million in one of the cases and about $200,000 in the other. There are a good many people, we suspect, who would trade a felony conviction, a $5,000 fine, probation and 200 hours of community work for $1 million. But Judge Gasch notes that 1) most of the money ended up in the pockets of GSA employees, 2) each of the contractors cooperated fully with the government in the investigation, and 3) the individual facts of each case have a bearing on the appropriate sentence.

Choosing the appropriate sentence can be the hardest thing a conscientious judge does. It is especially tough in white collar crime cases because two theories on which sentences are often based - rehabilitation of the criminal and the prevention of his doing violence to others - do not apply. There remain only punishment and deterrence, as well as that perennial question of whether the sentence should fit the crime or fit the criminal.

Whether a sentence is punishment enough depends on matters not normally part of the public record - such as, for example, the degree of culpability for the crime, the amount of cooperation after arrest, and the impact the sentence will have on an individual's personal and economic condition. Contrary to the implication in Mr. Silbert's memorandum, a jail sentence is not always regarded by a criminal as the most serious punishment he can get. Add to this the current pressure on judges to find sentences other than jail terms for many offenders and you can construct a legitimate rational for almost any sentence.

As far as deterrence is concerned, who knows what it takes to convince some goverment employees and the contractors they deal with to act honestly? Is it the kind of sentence imposed by Judge Gasch, the short jail terms requested by Mr. Sibert, the two-year term recommended by the Baltimore prosecutor in the most recent case there, or the five-year term imposed by the judge in the Baltimore case because he believes the contractor was one of the orginal architects of the whole scheme? We don't know, and we don't think any of those involved in this current controversy really know. That's what makes choosing sentences in cases like these difficult, criticizing them easy, and defending them almost impossible.