The "Forrest Gump" star -- who played Lieutenant Dan -- posted a tribute Tuesday to his son, Mac, who he revealed died last month. Gary says Mac passed away on Jan. 5 and was laid to rest on Jan. 23 ... this after he succumbed to chordoma, which affects the spine.

Gary says Mac was diagnosed with this cancer in 2018 ... the same year his wife, Moira, was diagnosed with stage 3 breast cancer. Luckily, she's recovered since and is in remission.

Gary posted photos of himself and Mac together -- including a shot that features his son in a wheelchair. He also notes Mac was heavily involved in his foundation, and had a love for music -- noting he even helped create an album with original compositions.

He writes of his son, "In sharing our story, we hope to shine a little bit of light on what has been a difficult time for us as Mac was truly a light for all of us."

Gary adds, "An incredible inspiration to those who knew and loved him, he faced his battle with grace, courage, and love. Even with one setback after another, he never stopped living and learning, creating, and giving, and loving. He loved all our family."

He ends his tribute post with this ... "Thank you, Mac. You did it. Resurrection & Revival will live on. And so will you. In our hearts forever. We were blessed to have you in our lives as son, brother, and friend...and we will miss you and love you for eternity."

Mac was only 33.