Gratis

adj.kind, joyfulformal

What it means

Provided without payment or charge.

Reach for it when describing a good or service supplied without payment

The museum offers gratis admission to local students on Fridays.

playful
on the house
a venue informally treats the customer to an item
not when Reserve on the house for casual hospitality; formal terms need zero-price wording.
everyday
free
plain language should state that no payment is required
not when Free can mean unrestricted rather than zero-price and may need clarification.
everyday
free of charge
zero price must be unmistakable
not when The phrase addresses price only and says nothing about eligibility or conditions.
professional
complimentary
a business provides something without a separate charge
not when Complimentary can also mean praising, and the cost may be bundled elsewhere.
professional
pro bono
professional services are donated for public benefit
not when Pro bono applies to skilled service, not ordinary free products or promotions.
formal
cost-free
no direct financial cost is the supported claim
not when Cost-free can hide time, data, obligations, or costs paid by someone else.

Gratis states price, not the absence of conditions, data collection, or indirect costs.

The definition and the example on this page are written here. The dictionaries above ground which meaning is being described, not the wording.