For U.S. collegian Ricky Charles put Grenada's Spice Boyz ahead with a penalty kick in the 12th minute, but Cuba quickly jumped ahead on goals by Joel Colome and Roberto Linares in the first half.
Coached by former England star John Barnes, Jamaica's Reggae Boyz advanced on goals by Oneil Thompson and Luton Shelton, his third in four games.
Jamaica is seeking its fourth Caribbean championship. The closest Grenada has come was in 1989 when it finished second to Trinidad & Tobago.
All four teams already had guaranteed their place in the 2009 Gold Cup.