Church numerals without functions
This is just a shorthand, leaving off some things that aren't really needed to understand the concepts. If you want your $7$ to be written as a function again, all you need are a couple more implicit lambdas. Here's how I'd write the Church addition with those lambdas in place:

$$3 + 4 = \lambda g . \lambda z . 3 g (4 g z) = \lambda g . \lambda z . 7 g z$$

But we also know that:

$$\lambda k . f k = f$$

Therefore:

$$\lambda g . \lambda z . 7 g z = 7$$