How did you install your Lazarus?
Did you use the 32bit installer from our homepage or sourceforge (both the same).
Then how did you compile to 64 bit, where did you get the 64bit compiler?
There is a cross compile add-on on sourceforge, and if you install this it includes a 64 bit gdb.
If you upgraded an older installation check the path to gdb:
$(LazarusDir)\mingw\$(TargetCPU)-$(TargetOS)\bin\gdb.exe
If it does not contain the variables, then it will not find the 64bit gdb, but you can change it to point to it.
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You can also go to our sourceforge "windows 64" folder, and look for "alternative gdb". There you can download a 64 bit gdb.