Did Myrtle's husband (the garage man) know that Tom was his wife's lover?It seems unlikely that Myrtle's husband would accept so readily Tom's...

In The Great Gatsby, no, Wilson does not know that Tom is having an affair with his wife.  A sister knows, but she keeps the information to herself.

Wilson goes to Tom to ask about the car, and Tom tells him that Gatsby is the owner of the car that hits and kills his wife.

Of course, Wilson isn't aware of quite a few things, the most important of which is that Daisy was actually driving the car when Myrtle was hit.

Tom gets back at Gatsby in two ways, then:  he manages to hold on to Daisy, and he also indirectly leads to his death by telling Wilson that Gatsby owns the car that hit his wife.