While I don't know the brand of makeup (and many makeup artists use a variety of different makeups, and I did not do her makeup for the film, I can certainly tell you how to achieve a similar look to that of Gemma's in the movie.
1. Moisturize. It will help with the glow she had. Let the lotion sit for a few minutes before step 2.
2. A very sheer base. I like Bobbi Brown skin foundation with spf 15. You can still see freckles through it, but it gives a balance to your face.
3. Conceal. But GENTLY. Apply it to blemishes, then under the eyes, and at the corners of your eyes. You can also add it around the base of your nose if you tend to get red there.
4. An off-white shadow all the way up to your brow bone followed by a light shimmery eyeshadow from the crease downward- she also had a little bit of shimmer/glitter happening at the inner corners of her eyes and around the lower lashes. If you have a taupe color, you can apply that last just above the crease. BLEND.
5. Liner was only on the top lashes, slightly thicker as it went toward the outer corners, and hardly any in the inner corners. It did not extend past her lid line. I'd choose a dark brown, draw a line and smudge it a little. Slather on the top mascara. Little, if any, lower mascara.
6. Eyebrows were a *key* to this look: I'd try a kit such as RedEarth eyebrow kit, or Anastasia. Pick a brow powder that matches your eyebrows, and then (starting from the MIDDLE of each brow- the arch) draw a line PAST the outer corners of your eye. Slowly work your way from the arch again inward- being careful not to make your eyebrows slope downward. You don't want to look angry or manly.
7. Pink blush. Not bronzer, not a warm chocolate or plum... a baby pink blush. Make a fist and prop your cheek onto it... that is where you need to put the blush. Not too close to your eyes, not too close to your hairline. Literally, just on top of and below your cheekbone, and in a circular area.
8. Pinkish nude lipliner- try Whirl by MAC. Fill your lips with it, then put a chapstick on top of it. It will give that matte appearance her lips had in the film without feeling dry.
9. Brush a translucent powder over your face. Cornsilk powder is good and cheap. You can get it at Wal-Mart.
And, let's face it, she has flawless skin, but it doesn't hurt to have a good skincare regimen: make sure your face is clean and moisturized every day. She also had pale skin in the film- SPF will keep your skin looking young- make sure you also use it down your neck as that is often an overlooked "age giveaway" spot.
Finished look: http://photogallery.filmofilia.com/data/media/76/clash_of_the_titans_49.jpg
Makeup artist for a few years, professional actor for 10 years, photographer.