How can I add more GB on my phone so I can download More apps

No, if you sync photos to iCloud that will not remove them from your phone. And if you then delete them from your phone they will also be removed from iCloud.

While you can enabled Photo Optimization that will reduce the size of photos on your phone somewhat, but they will still take a lot of space.

You can’t. An iPhone will never have more storage than it had when you bought it.

You've either got to opt in for iCloud storage options or upgrade to a higher storage option

thatsahilkabir wrote:

You've either got to opt in for iCloud storage options or upgrade to a higher storage option

Umm, no. Buying iCloud storage will not add any storage to your phone.

Syncing photos to iCloud to free up storage is a way to go forward in this case (that’s if they have large files in their gallery)