Rashi: They would rejoice if they find the grave (death).

Ibn Ezra #1: Some say that Gil is a motion of Simchah and for a mourning, like song ¹ .

Ibn Ezra #2: Some say that like the joy of those who go to rejoice, so will rejoice those who desire death [when they attain it].

Devar Eliyahu: One who attains what he desired, e.g. wealth and honor, has Simchah. Gil is for something that lasts a long time - "Gil Yagil Avi Tzadik v'Yoled Chacham Yismach Bo." A Tzadik always clings to Hashem; his level does not change. His father has Gil. A Chacham constantly adds to his understanding. His father has Simchah over his new Chidushim. "Nagilah v'Nismechah Bo" - Gilah is for constantly enjoying the radiance of the Shechinah; Simchah is for Chidushei Torah and revealing its secrets, without end. If the pleasure of the world to come was at one level, its pleasantness would be felt only when one first attained it, like pleasures of this world. Rather, we will constantly have new understandings in Torah and added perception of Hashem, increasing desire to cling to Him. This is Seudas Livyasan - Devekus. "Livyasan Zeh Yatzarta Lesachek Bo" - Sechok is sudden Simchah that comes to man. Our verse teaches that death will be Gil (lasting joy) for them; Simchah is for the moment of death.

Devar Eliyahu: Sason is when something reaches its perfection. Semechim b'Tzeisam v'Sasim b'Vo'am - when the sun finishes its course and comes to the west, it has Sason. The grave is the final desire of these people.

Malbim: One whose way is hidden (he is destined for evil), it is better that he die and rest from his afflictions, than for people to rejoice [over their good portion] in this world.