Let me say that I'm truly sorry for this bad news & at what should be a happy time.
While I don't know how you feel with your specific pet "Gringo" He looks like a loyal friend.
Over the last 5 years I've / We have had to watch 6 of our Cats slowly pass on to Heaven ( It would not be 'Heaven' without them & others 4 were from Cancer , I buried Fuzzy last July . It hurts these pets/ friends were over a decade old as well.
I was hard & has taken a toll on Me , Just make Gringo as Comfy & Happy as you can in his last days , Spoil him as much as possible, & I believe you will know just what to do when the time comes.
My best regards to you & your family.
Davers
Having put down two of my own dogs over the years, I have sincere empathy for your situation.
Keep the pictures and your great memories of 'Gringo' close; after the tears will come a smile.
Don.....