@Disoriginals
For a T-shirt print. Go and review the printing requirements of the company you intend to use.
Typically, the maximum print image area is 20cm x 20cm (10"x10") @ 300 pixels-per-inch (ppi) which will give a canvas size of 2360x2360 pix rather than your 640x400 pix. Even for a smaller logo, canvas size typically 1000x1000 pix
Start your image with those properties and you will find your text not 24 pix but maybe 400 or greater. With Ofnuts advice on anti-aliasing that will take care of the pixelation.

Is that Gimp 2.8? You should probably check that the display resolution is correct in
Edit -> Preferences -> Display -> Monitor Resolution Gimp 2.8 often gets it wrong, Gimp 2.10 is much better. Assuming that is correct. Want to see what it looks like printed? Set the zoom level to 100% and in the View menu untick
dot-for-dot (remember to tick back after viewing)

File format, .bmp? Again check with the printing co. lossless png or a compressed tif will give a smaller file size for transmittal.