Several ways:

 # gather the password into $P
 stty -echo; read P; stty echo; 
 for i in *.gpg; do echo -- "$i" >> ~/t; printf '%s' | gpg -d --batch --passphrase-fd 0 $i >> ~/t; done

 # gather the password into $P
 stty -echo; read P; stty echo; 
 for i in *.gpg; do echo -- "$i" >> ~/t; gpg -d --batch --passphrase "$P" $i >> ~/t; done

 d=$(mktemp -d)
 # gather the password into a file named `p`
 stty -echo; cat > "$d/p"; stty echo
 for i in *.gpg; do echo -- "$i" >> ~/t; gpg -d --batch --passphrase-file "$d/p" 0 $i >> ~/t; done
 rm -rf "$d"