Your phone must be carrier-unlocked to use a GigSky eSIM.
An unlocked phone can use mobile plans from providers other than your regular carrier.
The best way to confirm your phone is unlocked is to contact your mobile carrier or the retailer where you purchased it.
Check an iPhone
Go to Settings > General > About.
Find Carrier Lock.
If it says No SIM restrictions, your iPhone is unlocked.
Check an Android phone
Android phones don't have one standard setting for checking carrier-unlock status.
Contact your mobile carrier to confirm whether your phone is unlocked.
If your phone is locked, ask your carrier whether it can be unlocked before you use a GigSky eSIM.