Gmail may not have the security features that Protonmail has but Gmail does provide a more fuller suite of services that can be used across multiple platforms. For example Gmail has mobile apps, calendaring, etc while Protonmail has yet to mature to that level functionality.
Another limitation is that at the moment you need to wait for at least 3-4 weeks (if not longer) to get a Protonmail account.
You mis the whole point here. Protonmail isn't meant to be used as mainstream email application. If that was the case, it would offer a much better free service experience to compete with the rest.
Other than that, what do you mean with that you have to wait 3 to 4 weeks to get an account? You can sign up and start sending and receiving emails with protonmail within 3 minutes.