There are a few things you can do.
1) ditch the gravel, go bare-bottom and use small terra cotta pots for the plants, using top soil, and covering the soil with your gravel.
2) Keep the gravel, but still use potted plants as described above
3) put the fish in a bucket, drain most of the water, and mix in the eco-complete. I'm not sure how well this would work, I've never used the stuff.
4) buy some bags of top soil. Put all the fish in a bucket, and drain all the water. Scoop out as much gravel as you can, and put down the top soil planting the plants as you go. Cover each section with some of your gravel as you cover the bottom of the tank. This is how I have my planted tanks set up, and didn't spend a dime for eco-complete. Place a small saucer/plate on top of the gravel, and direct the water onto it to prevent disrupting the soil while filling it with water.