The major difference between angiosperms and gymnosperms comes from the .

production of microspores versus megaspores
presence or absence of alternation of generations
presence or absence of vascular structures
dominance or lack of dominance of the sporophyte generation
presence or absence of a protective covering over the ovule

presence or absence of a protective covering over the ovule

Explanation:

In angiosperms, seeds develop from ovules and are enclosed in the ovary. So, their is the presence of protective covering making the seeds enclosed.

On the other hand, gymnosperms have seeds that are borne on cones, so they are naked due to the absence of any protective covering over the ovule.