A gel food is any food that can be ground up or cut up or processed into small enough pieces that it can be suspended in a solution of gel - unflavored geletin such as Knox Geletin.
There are many recipes - I generally use the "clean out the refrigerator" type of recipe - including loads of fresh spinach, peas, dandelion leaves, krill and shrimp. I toss in spirilana, beans and even various fruits is some gels.
There are many lists of good goldie foods listed here - they will give you many ideas.