In AdWords, a gap in the market in is when there is high
search but low competition on a particular keyword.
Go to https://adwords.google.com/select/KeywordToolExternal and
we will talk you through it.
You will see there are two tabs - "Keyword Variations" and "The
Site-Related Keywords". You are interested in the first one, "Keyword
Variations".
What you would do is type in your main product or industry,
for example, "posters".
Here
is what we get (at
the time of writing):

So you can see that there are a few gaps in the market for
this industry.
For example, 3 from the bottom we can see that motivational
posters are searched
for by quite a lot
of people and there
doesn't seem to
be as much competition
as for other keywords.
If
you have some motivational
posters in stock
then great. Just
use the keyword
and put it in its own
AdGroup so you can
have a dedicated
ad with "Motivational
Posters" as the
title.
If you don't have any in stock then get some fast.