gdb core.17259
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This GDB was configured as "x86_64-unknown-linux-gnu"..."./core.17259": not in executable format: File format not recognized

(gdb) r
Starting program:
No executable file specified.
Use the "file" or "exec-file" command.
(gdb) q


Thanks & Regards
dayalan

man gdb:

-c or -core
Use file file as a core dump to examine.

So try:

gdb -c core.17259