We have used GG in the past. In December we used both GG and Amazon Prime Now. We were pleased with both.
From GG we ordered the things Amazon Prime Now did not carry as well as the few things we absolutely had to have even though they cost more with GG. In preparing for the trip we had stalked the Amazon Prime Now website for months and observved that almost everything in our cart came in and out of availability, frequently. We weren't willing to accept that instability for some thing, so paid the higher GG price. The things we did order from Amazon Prime Now were things we either could live without if necessary or that there was an alternate to [e.g. alternate brand that was acceptable that they also carried]. We had also compared Instacart but at the time it was the highest cost of the three so we discarded it as an option.
We are going again in December 2019 and are already planning our grocery order. I have made a spreadsheet with what we plan to order and have priced it fully a couple times at GG, Instacart (Whole Foods and Publix), and Amazon Prime Now, and checked those carts on their own many times for availability. Instacart is now competitive price-wise so they are in consideration for us. GG is usually but not always the highest price. Amazon Prime Now still has things going in and out of availability; Instacart does too but not as often. I figure we will probably end up using all three for our order as none of them carry everything we want and need, and the spreadsheet will help optimize the split.
SW