This is the kind of situation where appellate and post-conviction options need to be explored thoroughly, and they should be with the help of a qualified attorney who handles this area of law.
On challenging the conviction itself: if there were errors during the trial, whether in how evidence was admitted, how the jury was instructed, how witnesses were used, or how the prosecution handled the case, those errors can form the basis of a direct appeal. A direct appeal must typically be filed within a specific window after sentencing, so if that window has not yet closed, it is urgent to get an attorney involved immediately.
If the direct appeal has already been exhausted or the window has passed, post-conviction relief is the next avenue. This includes habeas corpus petitions, which argue that the conviction or sentence violates constitutional rights, and claims of ineffective assistance of counsel, which argue that the defense attorney failed to do their job adequately. New evidence that was not available at trial can also form the basis of a post-conviction claim in some circumstances.
On the sentence length specifically, a post-conviction attorney can research comparable cases and argue sentencing disparity, meaning that the sentence handed down was disproportionate compared to similar cases in the same jurisdiction. Judges rarely reverse their own sentences, but appellate courts can mandate reconsideration when the disparity argument is compelling.
The gang enhancement is worth examining specifically. There have been legal challenges and reforms around gang enhancement statutes in various states, and depending on the jurisdiction, that piece of the sentence may be vulnerable to challenge under more recent legal interpretations.