According to the manpage (an experiment), gdebi only accepts a single
file as argument.  Thus

    for i in *_all.deb *_i386.deb; do
    do gdebi $i
    done

works, but

    gdebi *_all.deb *_i386.deb

does not -- all but the first file is silently ignored.

Not only would it be convenient for gdebi to be variadic in this way,
an intelligent gdebi would allow dependencies to be fulfilled in the
correct order.  That is, if

    for i in *.deb; do gdebi $i; done

expanded to

    gdebi festvox-arctic-awb_all.deb
    gdebi mbrola_i386.deb

Then the first gdebi would (presumably) crash because
festvox-arctic-awb_all.deb depends on mbrola_i386.deb.  Specifying
both at once would (hopefully) allow gdebi/dpkg to "know" to install
mbrola first.