'Can someone explain what has happened to the GOP?'
You mean the GOP that now controls both the House and the Senate due to the historic, record-setting rejection / defeat of the Democrats in 2014, the same GOP - none of whose Presidential candidates are under investigation by the FBI for crimes under the ESPIONAGE Act or for Corruption OR is an admitted member of the Socialist Party?
On a serious note...
The GOP has been taken over by self-serving Washington Establishment GOP who care more about obtaining and keeping power, profit, and their positions/jobs than representing the American people - in other words, they have become Liberal Progressive clones. With the exception of a small few there is no difference between them and the Democratic Party clowns.
In 2014 the change in power had NOTHING to do with Republicans, as they naively claimed / believe. After Democrats rammed the minority-supported Obamacare down the throats of the majority of Americans who opposed it, after they declared Americans had no RIGHT to know what was in it until the edict had passed, after a nearly $1 trillion 7,000-pieces-of-DNC-benefitting-Pork-filled spending bill was passed into law, after over $6 trillion was added in only 4 years, after years of non-representative government, and after Obama injected himself into the 2014 elections by declaring the election was all about HIM - his policies, his agenda, his record, the people OUSTED, rejected, FIRED the Democrats. By DEFAULT the GOP took over the Senate to go along with the House.
Moronic, bloated, self-important GOP leaders like Boehner and McConnell declared the election was about them, that the people had given them a mandate. Once in control of the House and Senate they began acting exactly like the politicians the people had just ousted from power. They began giving Liberals / Obama everything they wanted. By doing so they have drawn the attention and ire or the same people who had just ousted the Democrats.
The current GOP nomination race - dominated by the anti-Washington Establishment GOP 'rebellion' - is reflective of / the culmination of all of this. Trump, for example, is the antithesis of 'Washington Establishment GOP. People are tired of the corruption, lies, 'edicts', non-representative government, politicians pandering to and making illegals and terrorists / 'refugees' a higher priority than American citizens...and politicians on both sides of the aisle haven't been / aren't listening. THAT is why such 'outsiders' - like Trump, who is giving voice to the anger and frustration of the American people - are dominating the GOP race. Jeb Bush, who I would describe as 'THE' Washington Establishment GOP candidate, is so far behind he should drop out...but refuses to.
So there we are...IMO.