Hello student as there's no such eligibility criteria of no gap year in between class 11 and 12 so yes you're eligible to appear in WBJEE if you fulfill the eligibility criteria as mentioned below -

Candidates must passed or appearing class 12 in physics chemistry and Mathematics with minimum 45 percent marks  from a recognized board

For more details refer to the link mentioned below - https://www.google.com/amp/s/engineering.careers360.com/articles/wbjee-eligibility-criteria/amp

Feel free to comment if you've any doubt

Good luck