Those are both highly gamified apps.
So maybe let's not play loose with meaning of words.
Gambling means 1 of 2:
"play games of chance for money; bet."
Investing is not a game of chance. I'm not making investing decisions randomly.
"take risky action in the hope of a desired result."
Investing could be gambling according to this definition (there are risky investment strategies) but on average investing in U.S. stock market is not risky.
If you abstain from doing risky things, you can have 8-15% yearly return by putting all your money in S&P 500 tracking stock.
And if you want to blame someone for people's risky investing, blame https://www.reddit.com/r/wallstreetbets/ (1.3 million people reading stories like "$35k -> $1.25 in 4 months") not Robinhood for making an easy to use stock trading app.
Just because TDAmeritrade has a very spartan UX doesn't mean people haven't made very stupid trades on it.
Robinhood is more agressive when it comes to margin calls, that actually protects inexperienced investors.