So, now, there are two ways to get GBWhatsApp app on your iPhone. Both of them requires JailBreaking your iPhone (Thus, voiding the Warranty).
GBWhatsapp is a modified version of the original WhatsApp but with better and amazing features. If you are using WhatsApp but not satisfied with the results then download GBWhatsapp. If you are using WhatsApp then you would be knowing that WhatsApp comes with some limitation and restriction that can be annoying but all these limits and problem are excluded in GBWhatsapp app.
Also, before performing anything on your device, make a Backup of everything on your Phone.
Then, after Jailbreaking, you can search for tweaks of GBWhatsapp and then proceed to install it.
you can also search for a tweak that allows running of Android Applications in an iOS Environment (This is not for all applications, because of obvious technical limitations as getting an Application do something which is not intended). Once, you have the tweak to emulate the Android Applications, then move further to download GBWhatsapp apk and then install on your iPhone.
And, boom... The required functionality is there on your iPhone.
But again, I seriously do not recommend doing so as this is incorrect, in many ways:
you know that
The developers spent so much time developing the Application, and you are modifying it, (Technically, hacking it).
If the thing that you wanted was a good feature, then it would have been implemented in the native WhatsApp application if it's not there might be a reason.
If you want to learn more about GBWhatsapp download then visit the official website.