I don’t really get why it should be compared to gambling when doomscrolling is literally just feeding yourself with negative news, and that will affect your personality (or maybe that’s your personality already). Gambling is different, it’s a form of entertainment, so we are happy when doing it - unless you’re addicted. That’s when you feel the negative effects, which maybe in the future ends up with you browsing news about yourself.
In fact, one of the common things between the two things mentioned in the OP is that the release of dopamine hormone, it is a very natural process of our body. But when we spend too much time on any thing or do something we like too much, the demand for this dopamine hormone increases even more, which results in our addiction to it.
In this case, social media and playing on gambling sites are both common things. So as you said, if we keep social media scrolling and gambling under control, there is no problem. The problem is only when things are out of control.