I’m not used to arrive/arriving so early.

[Barash]

Which one is supposed to be correct:

I’m not used to arrive so early.

or

I’m not used to arriving so early

and why?

 

[5jj]

I’m not used to arriving so early.

That's the one. 'To' is a preposition there; it is followed by a noun, pronoun or a gerund (-ing form).

 

[Barash]

So... In this case (I'm used to) "To" is always followed by gerund?

 

[emsr2d2]

So no ellipsis here in this case is "I'm used to" "To" is always followed by a gerund?

When it's followed by a verb, yes.

I'm used to eating breakfast at 5am.
She's used to being tired.
They're used to having lots of money.

 

[tedmc]

You say you are used to something (gerund/noun), not you are used to do something.

I am used to noise.
I am used to being late.
I am used to arriving early.