There are period returns for Off-Peak travel for both routes (via Staines or Slough). However, for travel at times when the Off-Peak Return isn't valid, you can only get an Anytime period return fare for travel via Staines. For travel via Slough you'd have to buy two Anytime singles - so in most cases it'd be cheaper to use contactless PAYG.
The closest stations to London which have Anytime period return fares priced by GWR are probably Swindon or Pewsey, being the closest "Intercity" stations. To a large extent this mirrors the fact that historically speaking, Network Southeast only offered Day Returns (either "open", i.e. Anytime, or "cheap", i.e. Off-Peak) or latterly Network AwayBreak tickets, which had time restrictions and were eventually merged with Saver fares to become Off-Peak Returns.
Only the InterCity and Regional Railways sectors of BR offered Open Returns (now called Anytime Returns). This reflected the fact that it was considered more likely that people making longer-distance journeys would be staying overnight.
Another consideration was that it would be unlikely to avoid ticket checks on an IC service whereas on a crowded NSE commuter train guards would often be unable to get through, if there even was one (many lines were converted to DOO by NSE).
The latter would have been linked to the risk of period return tickets being used multiple times as a quasi "season ticket" by passengers - particularly in the days when the outward portion of an Open Return was valid for a month.