Problems using :read with external command

I have a file, test.txt, containing two lines:

18415
480

If I do:
:%!gawk "{sum+=$1}END{print sum}"
I get the result in a new buffer, but if I try to put the result below
the last line, using
r %!gawk "{sum+=$1}END{print sum}"
I get:
E484: Can't open file test.txt!gawk

How can I put the result of an external command below the last line?
Many thanks in advance,

--
Cesar

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