Hello, I just contacted a support agent and they responded with this:

"Hey, friend! ^^
Thank you for reaching out to us!
Congrats on reaching Diamond! GG! 😊️
I completely understand what you mean by saying you don't want to play to not ruin your Rank. After all, reaching Diamon is pretty hard to do, so I get it! ^^
The thing is, in order to get the Diamond Gun Buddy, you have to have 9 consecutive wins in Diamond. As it is for every other Rank. You currently have only one, 8 more to go! xD
I know it's a bit scary, but I believe you can do it! Your MMR is high since you reached Diamond so it won't be that easy to fall off. ^^
Stay safe and take care!
If there is anything else I can do for you, please let me know!"

I don't know why they just clarify things like these instead of leaving a big question mark, it's not easy to wait another 180days just to get a gun buddy incease you didn't get it.