According to the T-Mobile website (source);

Are you staying in the EU? Then you can go on holiday with your mobile without worries. You use the calling minutes, text messages and MBs of your PrePaid subscription as you are used to in the Netherlands. Without extra costs. Pay the same out-of-bundle rate as in the Netherlands. See below which countries fall in the EU zone of T-Mobile. Are you visiting a country that falls outside the T-Mobile EU zone and do you still want to use the internet? Then activate a Travel & Surf pass. Check in advance in which zone the country you are going to visit falls.

 

Countries in zone EU
………...
Including Gibraltar
Czech Republic
Vatican CITY
United Kingdom
……...