I would not use scaling for this, the relevant bit for fonts is
The size of type, whether specified in pixels, points (1/72") or millimetres, is the height of an em-square, an invisible box which is typically a bit larger than the distance from the tallest ascender to the lowest descender.
I finally pulled from Windows, Calibri Light and say from the j descender to the l ascender is about 2 mm.
Remember Gimp works in pixels and if (I hope ) your label is at least 300 ppi resolution then 23 pix = 1.95 mm and 24 pix = 2.03 mm (2 mm not possible)
For the height of a capital letter = 2 mm maybe use a font height of 3 mm
The size of type, whether specified in pixels, points (1/72") or millimetres, is the height of an em-square, an invisible box which is typically a bit larger than the distance from the tallest ascender to the lowest descender.
I finally pulled from Windows, Calibri Light and say from the j descender to the l ascender is about 2 mm.
Remember Gimp works in pixels and if (I hope ) your label is at least 300 ppi resolution then 23 pix = 1.95 mm and 24 pix = 2.03 mm (2 mm not possible)
For the height of a capital letter = 2 mm maybe use a font height of 3 mm