can i get ban for using bubblestrat?

No.

As long as the bubblestrat defender you put in is on the same account as you train the gym up with, then you don't contravene the ToS.
This means you can gain fast XP, but you wil be left with a very low level defender in a highly trained gym.

However, if you use an alternate account to add a bubblestrat defender, and then use your 'main' to train the gym and place another defender, then that does contravene the ToS.

TL/DR Bubblestrat for XP is fine...alternate accounts for bubblestrat is not.