decompress, skip lines, delimit

Edit: actually, looking again at your issue + code (which is basically the same as mine), I think the main thing to change is to add a Columns field to your options record. When it's unspecified (as we are both doing in our M), I think it just looks at top row to determine column count (some number less than the ~20 you mentioned). So, I think the simple fix for you is:

Imported = Csv.Document(Decompressed, [Columns=20, Delimiter="|", Encoding=1252]), // <-- added Columns=#

Or, upstream, get the appropriate number of delimiters appended to first row such that it matches column count for primary data

Original post for references:

The following worked for me.

For reference, I made two simple csvs of format:

Zipped each with 7zip into gz. Note: original csvs in same folder but we are filtering them out per your procedure.

The following M successfully unpacks these.

let
    Source = Folder.Files( #"<FolderPath>" ),
    FilterGz = Table.SelectRows(Source, each [Extension] = ".gz"),
    AddGzParse = Table.AddColumn(
        FilterGz, "gz_tables", each 
        // steps for unpacking each gz - you can put these in a separate function if desired
        [ 
            gz = [Content],
            csv_binary = Binary.Decompress(gz, Compression.GZip), 
            csv_table = Csv.Document(csv_binary,null,"|"), 
            csv_table_trim = Table.RemoveLastN( Table.RemoveFirstN(csv_table,1), 1), 
            csv_table_promoted = Table.PromoteHeaders(csv_table_trim)
        ] [csv_table_promoted]
    ),
    // use our output from first row to automate column type and expansion
    // note, it's csv so all cols come in as text
    FirstTableType = Value.Type( List.First( AddGzParse[gz_tables] ) ),
    FixGzParseColType = Table.TransformColumns( 
        AddGzParse, 
        {{"gz_tables", each {_}{0}, FirstTableType }} 
    ),
    SelectMetaAndTables = Table.SelectColumns(
        FixGzParseColType,{"Name", "Date created", "gz_tables"}
    ),
    ExpandGz_tables = Table.ExpandTableColumn(
        SelectMetaAndTables, "gz_tables", 
        Type.TableSchema(FirstTableType)[Name] 
    )
in
    ExpandGz_tables

Output