The Gemini symbol (also called Gemini glyph) is a shorthand representation of the Gemini zodiac picture (see below). However, as with the picture itself, much is based both in interpretation and the preference of the writer, and can range from simple to extravagant.
The astrological symbol for Gemini represents the twins. Above are some examples of commonly used styles for the Gemini symbol. Each of these is equally valid - there is no 'correct' version as long as the general shape is maintained.
The word 'Zodiac' means circle of animals (though note several signs, such as Gemini aren't animals in the normal sense). These 'animals' were based originally on the pattern of the stars in their corresponding star constellation. Basically what the earliest astronomers and astrologers saw when they squinted at the night sky.
Due to this although Gemini will always be represented by twins in some form, there isn't any one 'correct' shape or design for this representation. It's a cosmic game of join the dots, and some constellations even have alternative ways to form the general concept from the dots (stars).
Any picture of the concept of twins can therefore be comfortably used here. Designs range from the space-age to the ancient, and are limited only by the imagination of the artist, as long as it gets the point across.
Below is a representation of Gemini the twins (and the Gemini symbol, inset) from a medieval wood carving from the 16th century. While it's not suggested this is a great picture (hey, you try carving something into the side of a tree which will look sexy in 400 years time!) it does show how the basic idea of Gemini the twins hasn't changed much over the centuries.
I have a separate article on Gemini tattoos.