Not really, that's a myth. Even though there's tight gun restrictions in some countries, it's not like the government took away the guns that were already in circulation or that you can't get guns at all.
See? Europe has a lot of guns. Not US-level, but still a lot.
The major difference is that there's no gun culture, and europeans don't use their guns as a toy like americans do. Folks aren't pround of having guns and it's not like anyone pretend these guns make people safer. A lot are used for hunting purposes only or are remnants of the world wars. Automatic weapons are also very rare when compared to the american counterpart.
All of this contributes for a smaller gun violence, mass shootings and accidents in general.
And when terrorist makes attacks, most of the time they will need to buy guns from Albanian/Bosnian/Kosovar contrabandists, which require funding and some knowlage of the underground black market, all of which reduces the chances of terrorist attacks as not everyone can become a terrorist unlike what happens in the US.
Edited 6/21/2016 13:34:14