Gem Squash is a smaller version of summer or semi winter squash the has a similar texture to spaghetti squash, but in serving sized fruit.
The vining plant is part of the curcubit pepo, so can easily cross pollenate with the corgette/zuchinni, of which I have a few so will not be seed saving and just buying new next year. I have this growing vertically behind the corn..
The fruit can be cooked and eaten at any stage. Picked small, roughly golf sized they can be roasted or baked and eaten whole as the skins are soft. Can also be steamed or bbq'd on skewers and doused in butter with salt and pepper.
Letting them get full sized, the skin will harden and keep a good way into winter. Boiled or baked, cut in half and butter salt and pepper. Some people ad marshmallows and make them sweet, but not for me!
They are really expensive at the supermarkets as normally they are grown as a pig feed here. There is only a really small market for food gem squash here so that drives up the price.