Hi @karane,
If you prefer to use button to filter the Gallery, then please follow the steps below:
1. Add three button, change the Text Property of the Button to:
"A-F", "G-M", "N-Z"
2. Under the OnSelect property of button A-F, enter the formula below:
Set(Var1,["A","B","C","D","E","F"])
G-M button OnSelect property:
Set(Var1,["G","H","I","J","K","L","M"])
N-Z OnSelect property:
Set(Var1,["N","O","P","Q","R","S","T","U","V","W","X","Y","Z"])
3. Under the item property of the gallery, enter the following:
If(!IsEmpty(Var1),Filter(Employee,Left(FirstName,1) in Var1.Value),Employee)
4. Under the OnVisible property of the Screen (this is used to Reset the filter):
Set(Var1,[])
Reference:

Regards,
Michael