Convert Mode and Scale

[Just Fred]

I need to do the following to all open tif images in Gimp.

change mode from indexed color to grayscale
scale image to half size (from 9912x7008 to 4956x3504 and resolution from 600 pixels per inch to 300)
save to xcf format using current file name plus a -300 suffix

I have a couple hundred files in 25 folders to do. Doing one at a time is slow going. Any way to speed up the process?

Thanks Fred