What does the apple in a garden represents? What differences would it make if it were a peach in an orchard?

The apple in the garden typically is the symbol for some kind of seduction, such as the one that happened in the Garden of Eden. A peach would, of course, make a difference in the meaning because the peach is typically a sweet, juicy type of fruit.

Some critics suggest that the apple symbolizes Blake’s creative work, which another of his contemporaries may have stolen and used as his own.

I think the type of fruit makes no difference, but the difference between an orchard and a garden would be the gleanings. Gleanings are meant to be shared. If we are indeed talking about Blake's work, a peach in the orchard could never have worked.