Whether Appellate Authority has power to condone the delay in filing of appeal filed beyond 3 months / 6 months

Yes. As per Section 107(4) of the CGST Act, 2017, Appellate Authority has powers to condone the delay beyond the period of 3 months or 6 months, upto a period of 1 month where sufficient cause for the delay is shown.

(ICAI FAQ PUBLICATIONS 06-09-2017 APPEALS AND REVISIONS: FAQ NO. 6)