Is GND filter function in ACR good enough?

[Sponge]

Actually, it can be done with one exposure and without the filter, with a black mask. It has been done in the past (yes, it was done with film too), and it is way easier to do now. It is the same technique as when dodging under the enlarger when printing a negative. What you need is a piece of black cardboard or if you are more picky, velvet, black velvet.

And don't get me wrong, this goes for long exposure only, so for landscape, you might have set up with a very dark ND.

You set up the camera to shoot, and you expose for the dark part under the horizon. During part of the exposure, you shade the top with your black velvet, moving it a bit up and down. The movement is to soften the edge of the shaded part of the image.

This reminded me of a video I saw a while ago using the Olympus E-M5's LiveTime function and the technique you describe but with what looks like a square filter. Here's the link in case anyone is interested.

[Giman]

You don't necessarily have to spend money if you don't want to. You say you like long exposure photography so I'm assuming you use a tripod in which case you can take two different exposures and blend them with software. It's not exactly the same but could be an alternative

Best for landscapes when a straight line of gradation impacts on subjects bisecting the horizon that shouldn't be darkened as much as the sky. Masking multiple exposures is a better option for some of these issues.