@fayda62 To 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/3521635
fantastic 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 15:00 (3 p.m.) on the day of your request.
Time / Day of request Earliest transfer day
Between 15:01 on Friday and 15:00 on Monday - Tuesday
Between 15:01 on Monday and 15:00 on Tuesday - Wednesday
Between 15:01 on Tuesday and 15:00 on Wednesday - Thursday
Between 15:01 on Wednesday and 15:00 on Thursday - Friday
Between 15:01 on Thursday and 15:00 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 3pm 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.