The officers of the CBI's anti-corruption branch sought to ascertain her financial accounts, sources said.
Gambhir is the sister of Abhishek's wife Rujira Banerjee who told the CBI earlier in the day that she will be available for examination in connection with the case at her residence in Harish Mukherjee Road between 11 am to 3 pm on February 23, they added.
Abhishek, the Trinamool Congress's Diamond Harbour MP, is the nephew of Chief Minister Mamata Banerjee.
The CBI had registered an FIR in November last year against the alleged kingpin of the pilferage racket Manjhi alias Lala, Eastern Coalfield Ltd General Managers Amit Kumar Dhar (of then Kunustoria area now Pandaveswar area) and Jayesh Chandra Rai (Kajora area), besides ECL Chief of Security Tanmay Das, Area Security Inspector, Kunustoria Dhananjay Rai and SSI and security in-charge Kajor area Debashish Mukherjee.
It is alleged that accused Lala is involved in the illegal mining and theft of coal from leasehold mines of ECL in Kunustoria and Kajora areas, sources said.