convert my timestamp dimension group from utc to gmt?

[mohiit_jain3]

I have a dimension group like this--

dimension_group: timestamp{
    type: time
    description: "day-month-year time of leave"
    timeframes: [
      raw,
      day_of_week,
      time,
      date,
      week,
      month,
      quarter,
      year
    ]
    sql: ${TABLE}."timestamp_at" ;;
  }

what method could I use to convert this time( in UTC) to GMT?

PS- The database which is connected to Looker in this case is Postgresql.