Hi Robert,
GPS satnav devices of all kinds certainly work in Spain, no problems.
I think the only thing you have to find out is if your Garmin device
has maps available for Spain, and it looks like it does.
One excellent (free!) option now available for all devices that can
use Google Maps (Blackberry, iPhone, Android phones, etc.) and have
incorporated GPS is the new Google Navigator option for Google Maps.
This is basically a GPS satnav program by Google and is available for
both Spain & Portugal. I've used it several times in Portugal and it
works wonderfully well. I wouldn't really recommend it for tourists
though, because Google Maps uses a data connection and unless you have
a special roaming data plan you may end up paying a lot for your data
costs.
But it's an excellent option if you have a special roaming data plan,
or if you have a local data plan.
Roger.