And it wasn't just on guns that the current party differs with Reagan. Reagan thought the rich should pay a larger percentage of their income than the middle class in taxes (calling it "fair"). And he did not hesitate to raise taxes when it was needed by the government -- or run a deficit when government spending was needed.
I'm not saying that Reagan was a liberal. He most certainly was not. But he was not so hung up on ideology that he refused to do what the country needed economically, and he did not hate those who disagreed with him so much that he could not compromise. Today's GOP may still say they idolize Reagan, but the truth is that they have left him (and other true conservatives) far behind in their race to far-right-wing fringe extremism.
Today's GOP no longer cares about what is good for this country (and its citizens) -- only about what they think Wall Street and the giant corporations want (so those entities will continue to fund their re-election campaigns). They are now the party of greed and self-interest.