In a rather enigmatic metaphor, Casy claims that systematic greed has taken hold in the country. He says that he once "used to give all my fight against the devil" but he now sees that the devil is not the only enemy afoot and, at that, not the worst.
Casy goes on to compare the new enemy to a Gila monster that has locked its jaw onto its prey. The jaw will not release even if the Gila monster is killed and all the while you try to pry loose the clamped teeth, poison seeps into the wounds.
Though Casy does not speak more specifically than this, his implication seems to be that whatever has taken hold in the country has taken hold everywhere and has taken hold of everyone. The problem is ubiquitous, systematic, and growing.
In the context of the chapter, we can reasonably surmise that this thing is related to corporate capitalism and its attendant values.
This negative effects of a burgeoning capitalistic and industrial system can be seen across the novel. The relationship between these effects and the people, like the Joads, is complex. We can see this in a line that appears later in the novel:
“(A preacher) says they’s wicketness in that camp. He says, ‘The poor is tryin’ to be rich.’ He says, ‘They’s dancin’ an’ huggin’ when they should be wailin’ an’ moanin’ in sin.’”
The people want to escape, fight against, and adopt capitalistic values. These values dominate the country and this is part of Casy's point.