Are gel based fumigation toxic?

Probably : It depends on what the chemical is and what you mean by toxic. If swallowed many fumigation chemicals are dangerous to people and animals.

Fumigation: Do you mean are chemicals used in getting rid of pests in a house toxic? Generally they are. Read any research related to the product used to know what you are dealing with. Better yet, find non-toxic ways to fumigate, e.g., citrus repels cats, marigolds does that for rabbits, etc. Modern mouse traps work quick and easily. Peace and good health. Avoid lawn products such as "Roundup."