First of all those who are looking for tests -- are you scoring "at least 50+" in previous year papers?
If not you should first do it -- you must be lacking some topic knowledge or English skills or problem solving skills. Being strong in Discrete Mathematics and C Programming is yet another required skill.
After you get to score 50+ (ideally 80+ if you have already seen the questions) then only going for tests matter.
Of all the tests for GATE I would recommend those where questions are from good University Exercises -- you can know this with the help of Google. But as told above; unless you complete the previous steps, tests do not really matter.
Standard book exercises are also like this - another icing in the cake. If you have build the cake, you can do it. You do not have to really worry about which book and all, this year we will add most of the questions in GO.