@abdul_khan429 bring your number over to giffgaff you will need a Porting Authorisation Code(PAC Code) from your old mobile provider.
Also, Before you can request to bring a number over to giffgaff if you haven't already you will need to complete the SIM activation process. You can follow this https://community.giffgaff.com/t5/Getting-started/Activating-your-new-SIM/ta-p/3521635fantastic guide on completing the SIM activation process
Once you received your PAC code and the SIM is fully activated follow this http://www.giffgaff.com/support/transfer link to the number transfer form, fill in the form with the PAC code and select the day you wish to have the number transferred over to giffgaff .
If you want the number transferred over the next day The PAC code MUST be submitted to giffgaff before 17:00 (5 p.m.) on the day of your request.
Time / Day of request Earliest transfer day
Between 17:01 on Friday and 17:00 on Monday - Tuesday
Between 17:01 on Monday and 17:00 on Tuesday - Wednesday
Between 17:01 on Tuesday and 17:00 on Wednesday - Thursday
Between 17:01 on Wednesday and 17:00 on Thursday - Friday
Between 17:01 on Thursday and 17:00 on Friday – Monday
NOTE: Requests made on Friday are processed on Monday. Requests made on Saturday and Sunday are processed on Tuesday. Also no transfers occur over public holidays e.g Bank Holiday.
As with SIM activation's number transfer process can take up to 24 hours to complete but usually 90% of transfers are completed before 5pm on the chosen transfer day.
This is what happens on the day you transfer - you'll experience a loss of service at some stage between the hours of approximately 9am and 6pm - typically this is no more than a few hours but can take up to a day, so please make plans so you're not caught short.
For a detailed guide on the number transfer process check out this http://community.giffgaff.com/t5/Getting-started/How-do-I-keep-my-current-mobile-number/ta-p/3521127 knowledge base link.