The biggest distinction between Black Cat/ DD and Gambit is "mirroring" vs "countering"
In High Elo, when you see Black Cat, you COUNTER more than you mirror => As a strategist for example, you pick White Fox.
But your team isn't forced to have a Black Cat as well, or force a hero pick to play with one. It would help of course because she's strong in the current patch, but it's not a must.
In Gambit's case, you're more or less forced to MIRROR if the enemy team has a gambit with rogue, starlord and sometimes Loki.
If you don't mirror, then they will overwhelm you offensively with every Gambit Ult.
You can say the same about other heroβs as well (dare devil) (black cat).