I will finish my BE in civil engineering next year. I'm planning to pursue my MS from US. Should I take a year gap to gain field experience or should I go forward with the master's immediately after BE?

Depends on what you want to do. If you want to do hard core engineering then higher study hardly helps and actually sometimes looked upon as a wrong decision. It makes you junior by 2 years, and you may or may not get the benefit of doing  Masters. Even if you get, it's negligible in most cases as far as I have seen. But if you want to stay in academics, then masters is a good choice. There is one more case, that you want to shift yor base in USA and there is only one way to do it, by studying there and getting a job there in any category - field/ academics. In either, I don't see any need why you would want to waste a year here gaining field experience. First world countries are much advanced in terms of construction techniques and the codes are also different. So better not to fool around the fields here. Take GRE, get a good score, get into a good college and board the plane :) Bon Voyage!

Its your choice. To be frank  the field experience will not improve or reduce your chances. After passing out I took a two years gap and did absolutely nothing. So far, out of 8 colleges I have applied, I have got 3 admits from top universities, 1 reject from a small university and 4 results awaited. If MS is your goal then pursuing it soon after BE might be good option if you are that person who says" wasted one year"....if you are one of those people who says I will travel relax for a year and take things lightly then that works too and also you can work in field and test out whether it is your cup of cake or not.