Nick Gordon -- the boyfriend of Bobbi Kristina Brown who was found legally responsible for her death -- has died in Florida ... TMZ has learned.

Nick's brother, Jack Walker Jr., shared the news Wednesday by writing, "GOD WHY I DID I HAVE TO LOSE MY BROTHER ON NEW YEARS," and adding ... "All I can do is cry." He later shared a tribute post with several photos of himself with Nick.

It appears Gordon was hospitalized for a short time before his death ... Walker says he was at his brother's bedside and spoke to him in his final moments. Reports say Gordon died of a suspected drug overdose.

As you'll recall ... Gordon and a friend found Whitney Houston and Bobby Brown's daughter face down in a bathtub on January 31, 2015, and she was placed in an induced coma. Brown suffered irreversible brain damage and remained unresponsive, and passed away in hospice care on July 26, 2015.

Bobbi Kristina's family sued Gordon for wrongful death, and he was found responsible for her death in September 2016 when he no-showed in court. He was ordered to pay her estate $36 million.

Of course, he was also under criminal investigation for her death when he was accused of domestic violence with his new girlfriend ... but the case was later dropped.

We're told Gordon had moved to Orlando a few months ago.