From everything I have been able to find out about this Spanish Prince, which is not that much, he was the most intelligent and gifted son of Charles III of Spain. It is apparently a historical fact he was his father's favorite son. He was the fourth son of Charles, and only one son away from being the King of Naples instead of Ferdinand IV ( 1751-1825 ) and two sons away from being the King of Spain instead of Charles IV. ( 1748-1819) His younger brother was apparently not too bright-- look at Goya's portraits of Antonio ( 1755-1817) and you will see what I mean. Why couldn't Gabriel have lived as long as these other three brothers? The Bourbon dynasty actually had the chance to have an intelligent and gifted individual to be King in either of these countries, but it did not happen. I find it somewhat unfortunate he had to wait until the age of 33 to be married when his two older brothers married at half this age. And, while I understand both the need for first in line heirs to the throne to perhaps marry earlier than younger brothers, and also the desire for Spain to have an alliance with Portugal, done partly through his marriage to the young Portuguese princess in 1785, ( unfortunately she was the product of yet anther one of those uncle niece marriages that turn my stomach when I find out about them ) it is unfortunate he was not able to marry earlier before he died in 1788, and possibly have more children who would inherit his intellect to survive him.
If he had become the King of Naples, he would have probably married Maria Carolina of Austria. I think from what I have read about these two princes, Maria Carolina would have been a lot happier with Gabriel than Ferdinand. Also, it is interesting to consider what an intelligent King of Naples would have done with all the challenges brought about because of the French revolution, Napoleon,etc. Or, if he had not become a king but just remained a younger Spanish prince, he still may have married one of the eligible princesses of Savoy or Saxony, or even Louis XVI's younger sister Elizabeth. While of course one would not have want Elizabeth to die of smallpox in the 1788 Spanish epidemic, perhaps she would not have- and look what happened to her anyway in France in 1794. And although Elizabeth and some of these other princesses may have been his first or second cousins through his mother, at least one of these alternate possibilities would have hopefully been somewhat better genetically than the Portuguese princess he did marry.
Gabriel was a music student of the Spanish master keyboard composer Antonio Soler the same as Ferdinand VI's' wife Barbara had been of Scarlatti. Soler had been a student of Scarlatti. You do not see Charles IV being able to do that do you?
After seeing Meng's portrait of Gabriel, it is possible, if Gabriel had lived past 1788, there could have been at least one more intelligent looking adult among the mostly unintelligent looking throng in Goya's famous painting of the family of Charles IV from 1800.