Which is better, to join TCS or to continue with GATE preparation?

Whether to join TCS or not, depends upon your economic condition. 

• If you have the financial support, you should concentrate solely on GATE preparation. With a full-time job, you may not have time to prepare well for GATE.
• Most people in the starting period at TCS earn around 22,000. With this amount, you cannot afford a comfortable living in metro cities. You will have to spend a major chunk of your salary on your rent alone, that too, if you are sharing a flat with a group of friends.
• The starting-level salary at a TCS job is not enough to make a comfortable living, let alone support your family. 
• With a salary of 22,000, you will be able to send a margin amount of around 2,000-3,000 to your family.
• You will lose precious time by commuting to and from work. Also, if you are living in shared accommodation, there will be disturbances with your studies.

It is difficult to do a full-time job and prepare for GATE. So, it is definitely better to prepare for GATE only and give your 100% to it, if you have the financial support.