Are there any examples of brainless animals (e.g. jellyfish) exhibiting seeking behavior, such as following smell or light gradients towards food, or following hormones towards mates?
Yes, there are plenty of examples! We refer to this seeking behaviour as taxis.
There are many types of taxis such as chemotaxis (attracted to chemical stimulus) or phototaxis (attracted to light). A complete list of taxis can be found on wikipedia > taxis.
For the purpose of this answer I will restrict to chemotaxis only
Chemotaxis (from chemo- + taxis) is the movement of an organism in response to a chemical stimulus.
Chemotaxis in Bacteria
Chemotaxis in Plants
Chemotaxis is also observed in plants such as rhibozium spp (Currier and Strobel 1976) for example
Chemotaxis in eukaryotes
Chemotaxis in animals that have "primitive" brains
Chemotaxis in "brainless" animals
Evidence of chemotaxis also come from Echinodermata (sea stars, sea urchins and others) (Miller 1985).
Echinodermata have quite definitely no brain!