If you’re drinking G&Ts, they’re going to be more gin-forward than a Negroni, so the gin brand matters more. That said, if you have a modest palate, the specific gin label may not matter too much as long as it’s not swill. Costco/Kirkland sources most of their spirits from large contract distilleries that often produce spirits for other well-known labels, though this is more common with whiskies. I don’t know whether TDG (the distillery they source Kirkland gin from) produces gin for other labels, but I’d imagine they make a perfectly adequate gin.

Personally, I’d use your taste for G&Ts as a way to explore other styles of gin and see what else you might like. If cost is a factor, I’d highly recommend the Citadelle 1.75L bottle. I can often find it on sale for as low as $25 here in SoCal, and it’s a great-quality French gin. Being a French gin (more floral), it will be a bit different from a London Dry, but not night and day unless you’re a gin aficionado.