I would suggest a Gatwick to London Zones 1-6 Travelcard, routed "Thameslink only". This costs £113.40 per week or £435.50 per month.
The optimal way to buy it would be to get a 'custom period' season ticket for a month and a few days, such that it covers 5 working weeks (i.e. the Monday or other first travel day in one week, to the Friday or other last travel day 5 weeks later).
The additional rate you're charged for the extra days is more or less pro-rata and that way, you avoid paying for a weekend's travel every 5 weeks. You can also time it so that you avoid paying for a season ticket during periods of annual leave.
Whilst KeyGo would charge the same weekly cost as a cap, it doesn't offer monthly or custom period season tickets. Also, KeyGo is only valid on GTR (Thameslink, Southern, Great Northern and Gatwick Express) services - so your wife would not get the full benefit of the fact she's paying for the cost of a Travelcard.
She would still have to pay on top for any journeys around London on the Tube, bus or on other rail routes - which her Travelcard would otherwise cover, if she buys it in advance rather than using KeyGo capping. Depending on how often those kinds of journeys might take place, that could be a significant factor.
If your wife is eligible for the 16-25 or 26-30 Railcard then the calculations may favour daily tickets instead.