I have not tried Gimp for that purpose.
You may check the uncompressed size in IrfanView: Image Information-> Current Memory Size.

I always upsample to 49-50 MB, and never had a rejection there.

AFAIK Alamy recommends using Genuine Fractals or Photoshop for upsampling.
You can download the trial version of Photoshop, it will work one month.

or try photoshop elements - i think same 1 month trial but it only costs $100 or so if you decide to buy it.

Hi, I'm new. Don't have Photoshop. Downloaded Gimp. Tried to scale pixels with it to get up to 48mb minimum for Alamy. Gimp software said that the images had been scaled appropriately but when  I tried to upload to Alamy , images were still rejected as less than 48mb uncompressed. Has anyone else used Gimp for this purpose successfully? Would appreciate help.

Make your photo 5025 on the long side and it will be large enough. Maybe you are looking at the wrong numbers when you resize with Gimp. It should work.

Irfanview will also work, it's free.

Elements is just fine.

Light Room is a photographers tool, and good for everything. You probably don't need Photoshop if you are going to convert from RAW, manipulate your images, correct color, keyword and organize.