Make enough C macro definitions and you can certainly do that, I did my final project in my high school programming class in the 90’s like that, made macros to simulate QBasic syntax and then just wrote it in basic, the end result is the macros converted everything into valid C++ and it compiled fine. Fortunately my teacher for that class was cool, and he was amused by it and since it compiled with no warnings and did what it was supposed to do, I got full marks for it.

Gmail wasn’t even the first, Hotmail, Yahoo mail, there were tons of free email offerings, even sites that would host your whole website for free like geocities. Gmail came into the market when 3rd party email being free was already well established. They just followed an Apple style of development, taking something that already exists and made a better version of it. Also back then their motto was still “Don’t Be Evil” and they mostly still kept to it, so they used that goodwill and the better user experience to grow it at a massive rate. And for the most part, its still the best experience for email for many cases.