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.