if you really wanted to. It's not like there's a test you have to pass.

For an example see Joe Lieberman in 2006 - https://en.wikipedia.org/wiki/Joe_Lieberman

Let her run.

Those monsters love her.

Republicans will vote for Republicans when the time comes. They only support Gabbard now because she is acting as a disruptor in the Democratic Party. One of the biggest Trump defenders I know voted for Sanders in the 2016 primary (he isn't a registered Republican, so he couldn't vote in their primary). If it were Sanders and Trump in the general, there is no question he would vote for Trump 100 times out of 100. Don't be fooled into thinking Gabbard will do anything other than drag down Democratic turnout just enough to help Trump and his 45% ceiling eke out another electoral college win.

but thanks.

There is a persistent idea that somehow either the parties control who gets on the ballot, or that there is some national system of elections.

Each state has its own rules for getting onto that state's ballot to run for whatever people are voting about.

You pay your fees, tick your boxes, or whatever it is you need to do in that state, and you are on the ballot. There is no party that decides you can or can't do it, there is no federal body which decides you can or can't do it, there are only the 50 state authorities who determine whether you have qualified by whatever rules they have.