Question

What did gulliver learn about the immortals of luggnagg?

Answer 1

Luggnagg has immortals, the struldbrugs. Perfectly ordinary parents can have them, and they are marked by a dot above the left eyebrow that changes color as they grow older until it hits black at age forty. Oddly, the struldbrugs tend to have normal, mortal children.

The problems with living forever as an old person include: 

The marriages never last – in fact, by law, they get dissolved automatically at 80 – because no two people could stand each other for eternity. 

According to the law of Luggnagg, the struldbrugs become legally dead at 80 and can no longer hold their own property. This is to stop them from taking over the world and holding it forever (3.10.22). But in practice, it also means that the struldbrugs have to beg for all time.

Language changes all the time. So, struldbrugs over the age of 200 generally can't understand the words of the younger generation, or even of younger struldbrugs.
HOPE THIS HELPS.

Answer 2

Gulliver was excited to meet the immortals of Luggnagg but when he met them he felt shocked and disgusted. He learned about the miseries and sufferings of these immortals. He learnt that these immortals suffered from many infirmities which come along with their age. They become incapable of friendship and dead to all natural affection. They want to be dead but the death was denied to them. So Gulliver felt disillusioned through the idea of immortality.