Gametes are always haploid, though the parent body may be haploid or diploid. How is it achieved in both kinds of organisms?

In haploid organisms, the plant body is a gametophyte and gives rise to haploid gametes by mitosis. The gametes fuse and produce a diploid zygote. Zygote represents sporophyte. It then undergoes meiosis to give rise to four haploid spores. Spores grow and develop into gametophyte. In this way the cycle continues. This type of life cycle is called haplontic. It is seen in Spirogyra, volvox etc. 
In diploid organisms, the plant body is sporophyte. Specialized sex organs are present in the plant body to form gametes (gametes represent gametophyte). Meiosis occur in the plant body at the time of gamete formation. Therefore, gametes are the only haploid structures in life. They fuse during fertilisation and give rise to the diploid individual or progeny and the cycle continues in this way. This type of life cycle is called diplontic. It is observed in gymnosperms and angiosperms.