Loans are hit-or-miss. The safe way is to keep him at Barça, teach him the proper way of playing and give him playing time according to his performances; that way, we'll have his situation under control instead of putting his fate in the hands of another club who are not interested in developing him anyway. If the next loan is another failure, he'll have more or less stagnated for 2 years in a row which is terrible for a player his age.
Some people are just able to adapt more quickly to new situations and for football players, that determines their ability to play well regardless of the circumstances (new country, language, coach, playing style etc.). Gerard strikes me as the type who relies heavily on a familiar and stable environment, much like Messi, and that's the reason why it would be a risk to send him on another loann, no matter to which club or league.