is it healthy to engage in these two activities excessively?
Whether it's healthy or not actually depends on how the individual engages in either of them, however, if there's anyone which can be said to be dangerous engaging in it excessively it should be gambling because if we're to count the damages that unhealthy gambling habits has done to indiscipline gamblers it grossly outweighs those of doomscrolling or whatever name you all call it.
No, it does not absolutely depend on the individual. Doomscrolling is by definition a very harmful behavior, it does not depend on anything.
It is always harmful. For gambling it depends, but to make the comparison more proper we have to assume an obsessive or addictive form of gambling. In this case it also does not depend on the individual at all, it is harmful. Period.
Gambling can be worse than doomscrolling. That is because when you gamble, you have a chance to forget everything because you feel enjoyment and excitement playing the games. You will spend more time because you think you can win but the fact is you lose your money. But doomscrolling can be worse if you cannot limit how long you use your mobile phone to browse. You will pass your day without doing positive things and that is laziness which you need to avoid.
I assure you, most people spend more time per day addicted to social media than people spend on gambling.