I'm not really a big fan of Pimm's No. 1, and I never have been. Because of this, I devised my own Pimm's replacement. It's much better than the original Pimm's, I assure you. Pimm's is an out-dated product, anyway. No one drinks it anymore, right?
1.5 oz London Dry gin
.25 oz sweet vermouth
1 dash triple sec
1 dash Angostura bitters
1 dash tonic water
Pour into a tall glass, fill with ice, and top with ginger ale. Garnish with a slice of lemon and/or piece of cucumber.
Now, a question: Did what I just write irritate you?
It should. Let us count the ways: (to clarify, what I wrote above is not my opinion)
1) What I made here isn't a Pimm's Cup. Why? Because regardless of the actual name of the drink, it specifically calls for Pimm's No. 1. My (perceived superior) approximation of Pimm's No. 1 is a different ingredient, and therefore the drink above is really only a variation.
2) My "homemade Pimm's" flavor deviates from the original more than your average brand-swapping of spirits in most drinks. The resulting flavor of my Pimm's Cup is very different than the original Pimm's Cup, and so both drinks should really not bear the same name.
3) My distaste for Pimm's No. 1 gives me no right to change the Pimm's Cup without changing its name, especially if I'm serving it at a commercial bar or issuing the recipe to readers. I owe it to my patrons/fans for my drink titles to accurately describe what they're getting, and I owe it to the annals of cocktail history to do my best to serve drinks as they were originally intended, and if I don't, then I should document/notate it as such.
If you agreed, then you should have no problem switching "Pimm's No. 1" with "Rose's Lime Cordial" and "Pimm's Cup" with "Gimlet" in what I wrote above. Replacing the Rose's with lime juice and sugar gives you a delicious drink, but you should not call it a Gimlet.
Just recently this discussion has flared up again when Michael Dietsch purposefully visited/flamebaited the subject and Doug Winship did it inadvertently.
Right now, a popular trend is to make your own lime cordial. However, pre-mixed lime juice and sugar in a bottle is not a cordial. (One of the more interesting lime cordial recipes is here, which uses agar to make the mix crystal clear. The clarification process changes the lime flavor into a more subdued note.)
Even if you do make your own lime cordial, still use caution in what you call a Gimlet. Regardless of how delicious you think your own cordial is, if it tastes nothing like Rose's, then perhaps your drink is a Gimlet variation. The Gimlet cocktail calls for Rose's Lime Cordial, not simply a lime cordial, just like how a Pimm's Cup calls for Pimm's No. 1, just simply a fruit cup.
If you find that Rose's has no place in your house or on your menu, then academically, it makes more sense to remove the Gimlet from your repertoire than to remove Rose's from your Gimlet.