Gabacho-gabacha in Spain is a term of scorn for the French. Gusano is a worm or grub (a mediocre brand of tequila has one in each bottle) but also a miserable SOB, etc. Terms change meaning with time, as they do everywhere. During the imposed reign of Maxmillian, gabacho would apply to one of the French or foreign occupiers. Later it remained a term of scorn- little white worm or take your choice. White because the majority of Mexicans, Mestizo or with Indian blood, are darker than the Europeans or Americans. But nowadays this is saloon talk.
NOTE: 99 44/100 of Mexicans are more polite than we generally are. I doubt that you would hear either of these words in a visit to Mexico. The ordinary Mexicans are very solicitous of anyone who is friendly and treats them fairly and as equals. The old politness especially prevails in the countryside outside of the big cities.