“Gregory XIV's bull of excommunication as a penalty for such wagers was never specifically rescinded, but was abrogated along with all canon law provisions associated with the Ius Decretalium in 1918.”

While technically speaking it doesn’t take a bull to be a mortal sin, discretion and a well formed conscience matters here. There’s also a huge difference between $10 among friends, and $20,000 among cardinals or princes. The original prohibition was most likely to prevent the latter.

>mortal sin

for that shouldn't it be damaging to somebody or to your own soul? How would betting damage your soul?