Hello aspirant,

would like to tell you that YES, You can get admission in phd after btech by giving GATE Exam. you can write GATE for getting admission for PHD'S . try to get rank below 1000 and hence there will be a good and best institutes like IISC /IIT.

Further, below are the listed requirements that you need to have for doing phd after b.tech from India :

• The average score in GATE exam or other equivalent national exam is mandatory for students belonging to Non Centrally Funded Technical Institutes (CFTIs). ...
• At least 8.0 CGPA out of 10.0.
• Only the student studying in the final year of BTech/ BE can apply.
• Well written SOP.

Further, you can read the below mentioned article to know more about the details on GATE exam : https://engineering.careers360.com/articles/gate

I hope that this will help.