Then this should also work:
#!/usr/bin/bash
/usr/sbin/sendmail -oi -t <<EOF
From: abc@xyz.com
To: def@xyz.com
Subject: GAP Report
Content-Type: text/html; charset=us-ascii
Content-Transfer-Encoding: 7bit
MIME-Version: 1.0
<pre>
$(cat "$FILE.txt")
</pre>
EOF