How to debug U-boot with GDB?

I downloaded the u-boot source code from https://github.com/radxa/rockchip-bsp.git.
Assign a value to CONFIG_TOOLS_DEBUG in u-boot/Makefile to enable the -g option.
But when I use gdb to debug uboot.img, the following prompt appears:
“0x7ffd64fc2910s”: not in executable format: file format not recognized