1. Connect to both SharePoint(SP) lists in Power Apps
2. If the button is outside of your gallery:
Patch(
SPConnection2,
Defaults(SPConnection2),
{
Col_In_SP_List2: Gallery.Selected.Value
}
)
3. If the button is inside your gallery (and the data you want to patch is selected in the gallery):
Patch(
SPConnection2,
Defaults(SPConnection2),
{
Col_In_SP_List2: ThisItem.Value
}
)