Hello PKL,
To import GUI on TouchGFX Designer, go to "Edit" > "Import" > "GUI"
But that would work only if your GUI is in the same version as your Designer.
So if you work with 4.21.3, you need to have your GUI updated to 4.21.3; to make it, very simple, open your GUI's touchgfx file with TouchGFX 4.21.3, and click yes when it asks you if you want to update.
Some functions have changed between the 2 versions you mention (see changelog since 4.18.1), so be sure to modify them in your project after importing your GUI, and don't forget to regenerate code on STM32CubeMX as well.
/Osman
