'will/be going to' confusion

In sentences like this, will is used for things that will happen as a matter of course. The view of the lake is there for all to see; the person only has to turn their head. Erik's phone call probably concerns the person being addressed, so it's only natural that the speaker will pass on his message.

I'm going to is more likely to be used when someone has made up their mind to do something, or is promising something special.

When Erik phones, I'm going to tell him just what I think of him for treating you like that!

You're going to see some wonderful scenery when we get into the mountains!