If dividend declines at a constant rate indefinitely, is Gordon growth model still valid?

Yes.

But isn’t the assumption of GGM constant growth?

If the dividends aren’t growing then the assumption fails?

Give it a try in Excel.

But isn’t the underlying assumption of the GGM that a dividend will grow in perpetuity? If you are assuming that the dividend is going to decline at some point in time, wouldn’t it just be some variation of a DDM and not necessarily the GGM by definition?

Edit: Just re-read the post and noticed the word *indefinitely*, I stand corrected.

Growth can be positive or negative.

Give it a try in Excel.