How can i remove gestyy from my Samsung mobile?
It's a advertiser/browser hijacker so to remove it you need an antivirus or antimalware app, there are plenty to choose from on the google play store.
Malwarebytes is highly regarded, but it's your choice which AV/AM you want to use.
https://play.google.com/store/apps/details?id=org.malwarebytes.antimalware&hl=en_GB&gl=US