Isn’t this slap a Gundam reference?

It absolutely is! But that’s not the first Gundam reference in Kaguya-sama: Love is War, nor is it the last. As far as I can tell the last one is the V-tuber plan notebook Papa Shirogane has. Image

As for the first, I think it’s when Karen sees Shirogane and Kaguya kissing at the end of season 3. I’m not certain though; it’s possible there’s another one even earlier.