Question

why is cycas placed under gymnosperms

Answer 1

because it is a plant of a group that comprises those that have seeds unprotected by an ovary or fruit , which are also its characteristics

Answer 2

Cycas are put under gymnosperms because like gymnosperms, they have naked unfertilized seeds directly fertilized by pollination.

One more similarity is the un-branched structure of stems.  

Moreover, in cycas plants, special roots called coralloid roots are present which are associated with Nitrogen fixation. Cycas plants have largely divided leaves which resembles with palms and ferns plants in overall appearance.