An alien species has 102 chromosomes in its diploid somatic cells. its haploid gametes will have chromosomes.

Diploid cells means they have the full sets of chromosomes, which are different for different species.
In opposite, haploid cells means they have half the number of chromosomes, usually due the cell division called meiosis.

So if the alien has 102 chromosomes in diploid condition, their haploid gametes for reproduction should be half of 102, which is 51.

This is necessary for gametes cells because when two gametes fuse together and form a zygote, the chromosomes number should be maintained as normal 102 instead of doubled.