Can you add a Grand Total in Power Query

Not sure how to fit this in with the rest of your query, but to add a GT row to the table you show in your most recent response, you just need to construct the row, then add it to the table:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjIwMlQIcFEwMFTSUTI1MTQ2MlOK1UESNwKKm5hbGplZooobg8QtDMwtTFHFTcDqDSwtjZViYwE=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Year/Period" = _t, #"LA Gateway" = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Year/Period", type text}, {"LA Gateway", Int64.Type}}),

//create grand total row
    gt = [#"Year/Period"="Grand Total", LA Gateway = List.Sum(#"Changed Type"[LA Gateway])],

//Add the row to the bottom of the table
    withGT=Table.FromRecords(Table.ToRecords(#"Changed Type") & {gt})
in
    withGT

You could certainly create a function using this algorithm, for example, and use it to add subtotals for different groups -- perhaps feeding it the Table and the column(s) to total as a variable