Open Code::Blocks Settings > Compiler and Linker Settings... and add the following under Global Compiler Settings > Compiler Settings > Other Options
`pkg-config --cflags gtk+-3.0`
and add this to Global Compiler Settings > Linker Settings > Other Options
`pkg-config --libs gtk+-3.0`
With the ` this has the effect of running the pkg-config tool
calling --cflags
gtk+-3.0
(or --libs) which, assuming
libgtk3-dev is installed, injects the necessary search folders and
lib directives onto the gcc command line.