Rare or not, are US federal guidelines and statistics available for
sentencing for "possession of a gun by an addict"?
As the solid answer by DM notes, U.S. federal sentencing guidelines are completely publicly available online.
There are no statistics, that are publicly available, at that level of granularity without personally combing through PACER for every relevant case and analyzing those records case by case yourself. You would be hard pressed to get anything more specific than "federal firearms offenses" (which make up about 14.7% of federal criminal cases). A review of that specificity would typically be undertaken by a professor, a research assistant and a student intern or two, as one of their major research projects for a full academic semester. It would involved hundreds of hours of quite skilled work.
There are statistics about the extent to which sentences actually imposed by judges are above, within, or below the guideline sentences, although again, not at this level of granularity. But, below guideline sentences are quite common for most offenses, while above guideline sentences are quite uncommon for most offenses. This is, in part, because one of the relevant statutes calls for a judge to impose the lowest sufficient sentence for an offense, and in part, because there are more circumstances to justify leniency than there are to justify an above guidelines sentence (as measured by the frequency with which such factors are present in a case).
Overall, about 1-2% of federal criminal sentences are above the guideline range, about 40% are within the guideline range, and the remaining 58-59% of federal criminal sentences are below the guideline range.
Under the particular sentencing guideline pointed out by DM (i.e. §2K2.1), there were 2,171 sentences imposed in the 2022 federal fiscal year, of which 1,133 (52%) sentences were within the guidelines, 19 (1%) were above the guidelines, while sentences below the guideline range were imposed in the balance (47%) of sentences imposed. (Source)
The slightly higher rate of within guidelines sentences for this guideline mostly reflects the fact that one of the applicable sub-offenses covered by it (possession by a convicted felon) has a mandatory minimum sentence by statute with only a few narrow relief valve options.
Finally, keep in mind that the sentencing guidelines are tools by which judges are assisted in deciding on a sentence following a sentencing hearing, following a conviction. But, a pre-trial diversion is outside this process and outside the jurisdiction of the U.S. Sentencing Commission. It is a function of prosecutorial exercises of discretion, regulated only lightly by U.S. Justice Department internal policies, and not something in which the judge has any meaningful discretionary role. Sentencing statistics only show you the "shadow of going to trial" or plea bargaining that provides a context of pre-trial diversion negotiations and discussions. Some discussion of pre-trial diversions can be found in a 2016 report on the subject. Pre-trial diversions are used with any significant frequency in only a minority of federal judicial districts.
All of this said, however, the nuances of how federal prosecutors handle particular kinds of cases and the factors that are considered by them and influence their decisions are best answered by people with a wealth of experience working in the federal criminal justice system as a lawyer. Statistics alone don't tell the whole story.
The main skill set that lawyers who practice criminal law gain from on the job experience, which is really what you pay them for, is an ability to evaluate what sentences or alternative sentencing options are viable in the facts of a particular case. This skill is used in every single case such a lawyer handles. Other skills, like familiarity with the rules of evidence, only come into play in one case in fifty or so in the federal criminal justice system.