a headstone for your gigi is a lovely idea! I also saw the ones at PetsMart and thought they were beautiful - flat marble stone with inscriptions. Have they stopped carrying those? Perhaps their online store has them??
Wish I had some sources for you - sorry!

Our two RB kitties - Ernie and bert, are also buried in out yard - right outside of our bedroom window. My hubby made cat shaped wooden headstones and then painted their names and dates on them. They are coated with weather-seal and holding up quite well. We placed a border of rocks and there is a little tree planted right next to them and I love having them there

Becky (craftlady) made a lovely garden for her RB Buterscotch and can probably direct you to a source for a stone since the garden she made has a very nice one!!

I know you are missing gigi and having a hard time. It does get easier but never completely goes away. Eventually, there will be days when you can think of her tenderly and smile - but it will take awhile.