You control either Giana or her sister Maria. Giana has a strange dream while asleep... She finds herself in a strange mysterious world, where everything is completely different. Gravity has almost disappeared - sometimes one feels like flying away - and everywhere there are unexplainable buildings and structures. Old grottos and deserted castles seem to hide lots of secrets, and frightening and hideous creatures appear. This wouldn't be too bad, except that Giana can't leave this world unless she finds the magic, huge jewel.
While searching for that, you collect blue dream crystals in order to raise your stamina (suffering from the evil creatures on your way) and also to be able to success to the next level. The way you collect those crystals is similar to the Mario series: jump against these rocks from below, and these will give you the crystals (most of the time). Note that sometimes the rocks hide enemies which will go against you once they are free! Enemies can be defeated by jumping on them or shooting them after obtaining the relevant power-ups.
The "Fire Wheel" transforms Giana into a punk with the ability to crush rocks by jumping beneath them. "Lightning Bolt" will award Giana "Dream Bubbles", a single projectile shot. "Double Lightning" gives her ability to shot recoiling projectiles. "Water-Drop", which protects Giana against fire etc.
Now, due to its huge similarity with the Super Mario game in terms of level design and gameplay, Nintendo pressured the makers of The Great Giana Sisters with warning of legal action should they not take appropriate action, and the game was voluntarily withdrawn from sale.
Graphics are cute (almost identical to the Mario series) and colorful although missing background detail. Well for its age (back to '87) it's pretty much good though. Sprites move smoothly on screen which makes the game very playable. As far as the sound, the C64 version offers wonderful tunes due to its SID chip. The game has plenty of music scores and cute SFX.