Gar roe toxicity has been recognised for well over a century and confirmed by feeding trials in which mammals and birds given gar eggs became ill. The toxic principle is a protein-associated ichthyotoxin present in the eggs but not in the muscle, which is eaten without harm across the southern United States. Cooking does not reliably destroy it.
Chemical protection of eggs makes sense given how gar spawn. Females broadcast large, adhesive eggs onto flooded vegetation in shallow water during spring high flows and provide no parental care whatsoever. An unguarded egg mass exposed for days in shallow, crowded water is an obvious meal, and a toxin substitutes for the guarding that species such as bass provide.
Which organisms the toxin actually deters in the wild is not established. The documented effects are in mammals and birds, and whether the fish and invertebrates most likely to encounter gar eggs are equally affected has not been well tested.