Symbolism, Imagery, Allegory
Gabriel Oak spends quite a bit of time in this book taking care of sheep and lambs. And do you know which other figure tends to do the same? That's right, Jesus. In this case, Hardy uses Gabriel's shepherding of lambs to show that he's a very compassionate, decent man with good Christian values. Just one look at a passage like this can tell us as much:
"Their noise increased to a chorus of baas upon which Oak pulled the milk can from before the fire, and taking a small teapot from the pocket of his smockfrock, filled it with milk, and taught those of the helpless which were not to be restored to their dams how to drink from the spout." (15.85)
On top of that, he also tries to use his compassion as a point of connection with Bathsheba, which he tries to accomplish by giving her a lamb. As he says to Bathsheba's aunt, "I've brought a lamb for Miss Everdene. I thought she might like one to rear: girls do" (4.18).
The funny thing here, though, is that by claiming that girls like to raise lambs, Gabriel also makes himself sound girly because raising lambs is something he does, and enjoys. But make no mistake. Gabriel isn't just some softy. He also gets down to business when he needs to, sometimes in really grisly ways, as we find out in the scene where he uses one dead lamb's skin to make another lamb feel warmer: "Gabriel had skinned the dead lamb, and was tying the skin over the body of the live lamb in the customary manner" (18.15).
In other words, Gabriel is good to the sheep and their lambs, but mostly insofar as these animals are worth money to him and his employer. In this sense, lambs show that Gabriel is really a multi-faceted character: he's compassionate when he needs to be, but also practical when he needs to be.