I don't know what Gab is, or what its typical content is like, though I'm getting an idea from the replies to this tweet. But the principle of free speech has nothing to do with the right of a company to use Paypal's services. It's to do with the right of individuals to say what they like without government incarcerating or otherwise harming them or their ability to speak.
If an entity stifles speech that entity is against Free Speech.
If your belief is that PayPal doing business with Gab is free speech, you have no idea what free speech is.
PayPal isn't the government.
A phone company isn't the government either, but I am pretty sure Samsung is not allowed to deactivate my phone because they did not like my speech.