Is there a better way ?

Yes, don't use it. Just cut some drywall to fit and then use regular
pre-mixed joint compound to set it. It's a garage, right? So it's not like
it'll have to be perfect. The only complication might be if there's not
enough points to anchor the patching piece of drywall. As in, some nitwit
cut the slot along the middle of a joist run or something. That being the
case you'll need to add in some sort of supports. A simple piece of 1x3 put
inside the slot, across it, will then let you screw through the old drywall
to hold the wood in place and then in through the new pieces. Then it's
just a matter of slapping on some joint compound, taping it, followed with
another coat and then some sanding.

Just remember, if the average contractor can do it, so can you.

Does it work, etc. ?

Please see the link to it:

http://paint-and-supplies.hardwarestore.com/50-276-tapes-beads-and-patches/wet-and-set-5-x-9''-drywall-repair-roll--611775.aspx

BTW: Not competent in plastering: what type and brand of plaster (or is
it
called spackle ?) should I use ?

Thanks,
Bob