I am unfamiliar with Xcode but if you can compile the code at the command line and have pkg-config working then check where the include files are that you need and add them to your Xcode development environment. Try
pkg-config --cflags --libs gtk+-3.0
at the command line for your includes and libraries.
From: Evan Wong via gtk-app-devel-list <[hidden email]>
To: gtk-app-devel-list <[hidden email]>
Sent: Fri, Jul 20, 2018 4:54 am
Subject: Use Xcode to run Gtk code instead of terminal