It's true that by default planyo works with gross prices. The tax rate (VAT / sales tax) can be defined in Site Settings / Company Info or in the invoice settings. You can, however, work with net prices when setting your pricing in Pricing Manager or when displaying the price to your customers.

Pricing manager

All the prices defined in Pricing Manager are interpreted as gross prices. If you'd like to enter the base price and the prices applied by your rules as net prices, you can do this without a problem -- you'll just need to add one last rule (must always be the very last rule) of type Always true with the price set to +X % where X is the tax rate (e.g. +7 %). This way the price calculated by Pricing Manager is gross (as required by planyo) but still you set the individual amounts as net prices.

'Your rate' box in the booking form

In Settings / Reservation form layout / Reservation form info box (direct link) you can change the default way the price is presented. Choose e.g. Custom and display the net price using the tag $(net_price_quoted).