Enough for what?

It is enough to surely get 30-50 marks but that won't get you even to an NIT unless you have some reservation

How to solve?

Getting answer to the previous year GATE questions takes you nowhere as in GATE questions RARELY do get repeated. But in many topics, same type of questions get asked. One example is cache memory from CO & Computer Architecture subject. If you know properly to solve one question from this topics, you can solve many similar ones. And if you are seeing someone's solution, make that your own by really understanding each and every step. You should never byheart someone's solution as in most times they will just backfire in GATE.

I know to solve all previous year GATE questions. Will I get into top 100?

If you really know the proper way to solve the previous GATE questions you have a high chance of getting into top 100. You should be able to get at least 80 marks from them and usually 60+ is enough. But this means you shouldn't make any mistakes while reading the questions and interpret all questions properly- no space for silly mistakes. To get into the safe space of 90+ you have to understand the topics properly. One example is Database Normalization. If you know how to identify the keys from Functional Dependencies and able to identify the Normalization level, you can answer most (almost 90%) of the questions from this area. But to guarantee 100%, you must need to understand what is normalization and why is it done, what is the significance of Functional dependencies etc.

Understand the concepts- what does this mean?

One example is "what is the complexity of quick sort?". If you know it is $O(n\log n)$ then it is not enough for GATE. You must know how that $\Theta(n \log n)$ is a better complexity representation for quick sort and how that came. In short those making GATE questions know that people will be by hearting this complexity and they will make sure the question is twisted and only those who knows how to derive the complexity can answer that.

Can I understand all concepts?

Probably no. But do as much as you can and especially do understand the concepts in Algorithms and Data structures- they are the subjects where the questions are usually twisted. I have seen many people leaving Theory of Computation for preparation. But that is as stupid as it can get. Usually they are asked quite a bit for GATE. And the questions are easily answerable if you have studied till Turing machines and reduction. Practise do help in TOC.

Enough for what?[edit]

It is enough to surely get 30-50 marks but that won't get you even to an NIT unless you have some reservation

How to solve?[edit]

Getting answer to the previous year GATE questions takes you nowhere as in GATE questions RARELY do get repeated. But in many topics, same type of questions get asked. One example is cache memory from CO & Computer Architecture subject. If you know properly to solve one question from this topics, you can solve many similar ones. And if you are seeing someone's solution, make that your own by really understanding each and every step. You should never byheart someone's solution as in most times they will just backfire in GATE.

I know to solve all previous year GATE questions. Will I get into top 100?[edit]

If you really know the proper way to solve the previous GATE questions you have a high chance of getting into top 100. You should be able to get at least 80 marks from them and usually 60+ is enough. But this means you shouldn't make any mistakes while reading the questions and interpret all questions properly- no space for silly mistakes. To get into the safe space of 90+ you have to understand the topics properly. One example is Database Normalization. If you know how to identify the keys from Functional Dependencies and able to identify the Normalization level, you can answer most (almost 90%) of the questions from this area. But to guarantee 100%, you must need to understand what is normalization and why is it done, what is the significance of Functional dependencies etc.

Understand the concepts- what does this mean?[edit]

One example is "what is the complexity of quick sort?". If you know it is $O(n\log n)$ then it is not enough for GATE. You must know how that $\Theta(n \log n)$ is a better complexity representation for quick sort and how that came. In short those making GATE questions know that people will be by hearting this complexity and they will make sure the question is twisted and only those who knows how to derive the complexity can answer that.

Can I understand all concepts?[edit]

Probably no. But do as much as you can and especially do understand the concepts in Algorithms and Data structures- they are the subjects where the questions are usually twisted. I have seen many people leaving Theory of Computation for preparation. But that is as stupid as it can get. Usually they are asked quite a bit for GATE. And the questions are easily answerable if you have studied till Turing machines and reduction. Practise do help in TOC.