Features causing hemlocks to be "typical gymnosperms" include the fact that its leaves are needlelike and evergreen, and it produces cones. The same can be said about most of the best-known gymnosperms, including pines, spruce and firs.
However, these features are not what make a gymnosperm a gemnosperm. To properly define what a gymnosperm is, we have to get a little technical and speak in terms of flower and fruit anatomy. All gymnosperms share this one feature:
Their female sex germs reside in ovules, as in regular flowers, but the ovules themselves are not enclosed within the flower's ovaries, as they are among flowering plants.
In our fruit section we say that, by definition, fruits are the structures that develop from maturing flower ovaries, and seeds develop from ovules inside the ovaries. Therefore, since gymnosperms have no ovaries, they do not produce real fruits, at least not in the botanical sense. Because no fruit tissue surrounds gymnosperm seeds, the seeds are said to be "naked." When early scientists wanted to express the term "naked seed" using word roots from classical Greek, they chose gymnos, which means "naked," and sperma, which means "seed," and came up with "gymnosperm."
Most but not all gymnosperms shed their seeds from scales held together in cones, not regular fruits. The picture at the right shows a cone of the Loblolly Pine, Pinus taeda, growing right outside my door. The three small, pale items on the left of the cone are the seeds, which drop from the cones when the bracts open, as they are here. The cone is about five inches long. A few gymnosperms, such as yews and ginkgoes, produce seed-bearing structures that actually do look "fruity." However, if you were to watch the seed-bearing structures of those plants as the ovules develop into seeds, you'd see for yourself that the seeds do not end up inside fruits. The fleshy parts which cause the seeds to look like fruits are actually modified seed coats -- part of the seeds themselves.
At the left you see another kind of gymnosperm cone, this one produced on a member of the Cycas Family, the Zamiaceae.
Most but not all gymnosperms are evergreen, and most do not possess regular flat-bladed leaves. At the right you see the magnified tip of a Redcedar branch showing green scales typical of the Cypress Family, a gymnosperm family. You may also know about pine needles and the stiff single needles covering the branches of spruce and fir trees. All of these are photosynthesizing counterparts of leaves found on non-gymnosperm trees such as oaks and maples. The sprout at the left is about one inch long (2.5 cm) in real life.
Answering "what is a gymnosperm" in terms of plant classification is actually trickier nowadays than it used to be. Just within the last few years advances in the study of plant evolution (particularly using genetic sequencing) have brought about a revolution of thought about how plants are related to one another. Nonetheless, many specialists would agree that members of the following plant families are Gymnosperms:
To delve into the gymnosperm world in more detail you may want to look at these very in-depth books available at Amazon.com: