I would absolutely give them a gift, its not like your wedding was local, they came, ate all your food, drank your free drink and then left. They took time off work, spent money on flights and accommodation and spending money. Their presence should have been enough of a gift especially as it was abroad. They obviously care enough about you to do all that to be there for you on your day.
If their wedding isn't abroad you should give a gift, these people are your friends and it comes across petty saying your not giving a gift cause they didn't.
If their wedding is abroad then don't give one as you like they did will spend money on all the above.
-Not relating to your post but posts in general about getting presents, its quite sad how grown adults get upset/pissed off about not getting presents. I would always give one but I'm not bothered if people don't give us one.
Enjoy your day and enjoy your friends and family, everything else is inconsequential!