@SumanthDundi7
A Gallery always has something selected! You cannot make nothing selected by default.
If you want to avoid that, then you need to put your own selection action in place. Meaning, use the OnSelect of the gallery to assign the selected row to a variable. Then utilize that variable (instead of the ThisItem.IsSelected) to determine a selection.
For example, very common to use the TemplateFill property to change the row color:
If(ThisItem.IsSelected, Yellow, White)
Instead, utilize a variable of the record in the OnSelect - Set(glbRecord, ThisItem)
Then the TemplateFill would become:
If(ThisItem.ID = glbRecord.ID, Yellow, White)
And...never refer to yourGallery.Selected - as it will not be what you want. Instead refer to glbRecord for the selected record.
I hope this is helpful for you.