I think a number of things may be possibly going on.. its all rather complex!
Firstly that it is certainly true that those who may be characterised as 'far right' are indeed a threat to many groups including people who are transgender, lesbian, gay, refugees, migrants, muslims etc through intolerance & bigotry. This would include physical threat.
The MPs who have spoken out so far seem to be mostly to the political right of the Conservative party eg Norman Tebbit, Jacob Rees Mogg, David Davies though this seems to be more attributable to their position re free speech and / or support for traditional gender roles.
It may be that in the overlap with free speech, press freedoms 'common sense' some political capital could be gained by those politicians, commentators etc
I also wonder to what extent conflating those who are gender critical with this position affects those people who rightly condemn the intolerance of the 'far right' and are new to the discussions around sex and gender. This can be powerful and persuasive.
It can also be a way of shaming (or attempting to shame) those who are left wing / progressive.
Some people may also genuinely regard views which do not affirm trans rights ideology as being 'far right' if this term means intolerance to them.
So perhaps its both descriptive and signalling.
There's clearly more going on in the targeting of GC twitter users, just as with the focus on Mumsnet or lesbian groups.