One of the two valid sentences begins with ydyn...?, the other one with dyn...?
Are both forms correct when asking a question?
I'm having trouble parsing am gyrraedd.
It is one way of expressing a future, often a near future. It is not especially common, but you will come across it from time to time, especially in spoken Welsh.
Something to be aware of rather than to use, unless you are with other people who use it.