@ Clueless - Please tell us which Gimp you are using - it does prevent guessing: Gimp Help menu -> About
If you want to try printing from Gimp set up an empty template based on the Gimp A4 landscape, which is 3508x2480 px @ 300 ppi . Then crop down according to the margin settings shown in File -> Print Setup. I do not know what your Epson uses, might even be borderless. but my Brother looks like this.
210 x 99 mm is half a A4 landscape, so you should be able to get two images out of one sheet. Using the cut-down template the values to check are the printing resolution - should be 300 ppi and the margins - should be zero.
If you want to try printing from Gimp set up an empty template based on the Gimp A4 landscape, which is 3508x2480 px @ 300 ppi . Then crop down according to the margin settings shown in File -> Print Setup. I do not know what your Epson uses, might even be borderless. but my Brother looks like this.
210 x 99 mm is half a A4 landscape, so you should be able to get two images out of one sheet. Using the cut-down template the values to check are the printing resolution - should be 300 ppi and the margins - should be zero.