How do I get more GB on my iPhone

you cannot increase the capacity of your iPhone.

you can purchase more iCloud storage, but you should understand the difference

What's the difference between device storage and iCloud storage? - Apple Support

Buy more iCloud storage - Apple Support

If photos are taking up the majority of your storage space, you can utilize iCloud storage and iCloud Photos to reduce the space your photos take up using the Optimize Storage switch

Set up and use iCloud Photos - Apple Support

you could also transfer the photos off your device and delete them from your phone

Transfer photos and videos from your iPhone, iPad, or iPod touch - Apple Support

or you could look into google photos and other similar apps

Sell it and buy a phone with more GB. That is the ONLY way.

You can not add storage space to an iPhone.