Glucose-Fructose Syrup

This stuff has started turning up everywhere recently. Sometimes it
says (from wheat), although it usually doesn't list wheat/gluten as an
allergen; sometimes it says (from maize); sometimes it is parenthesis-
less.

I have noticed a couple of times that I've drunk cordials with this in
because it isn't listed under allergens - Waitrose particularly bad
for this - but then had small symptoms that day, which made me
suspicious.

What is it? Is it safe, like wheat starch can be safe? What is the
default - i.e. if it doesn't say wheat is it more likely to be derived
form wheat or maize? It really is all over the place, and with very
little run-in time and if they're suddenly replacing sugar with
GlutenSugar I shall be very miffed!