Will it be good to attend the interview in government organisation ?

Waste of time - if the offer is half of what you expect, while there may be a way around the base offer, you certainly aren't getting anywhere near what you want (and in fact, you probably won't find anyone in or out of government willing to offer twice your salary, but good luck anyway).

Whatever they say they'll pay you be prepared to set aside some for taxes since they don't take it out of your pay. That's how they artificially inflate the figures and make you think you're getting paid more than you actually are. Then it's a huge, complicated, inconvenient process to figure out WTF you have to do at tax time. These people are just hoping and praying that you screw up so they can hit you with all kinds of penalties.

The amount listed in the advertisement is exactly what the job pays. If you are willing to accept that amount,, attend the interview. If you are not willing to accept that compensation, you will wast both your time and theirs by interviewing. The pay rate will not change. It was set at the time the job order / opening was created and approved.

Why would you expect twice your current salary for any job? Government pays the lowest wages they can for most jobs, including contractors. Continue looking elsewhere.

No. You get exactly the salary advertised. Government is 'open' meaning you are going to get exactly what is printed.

Waste of time to attend the interview. They cannot change the pay scales.