Software free of financial cost. For software that includes the source code, [open-source] is the best tag. Software can be either [gratis] or [open-source], or both. Only use this tag if "gratis" is a must-have requirement.
Gratis software is free of any financial cost. As such, it is often called freeware.
Even though the developer of gratis software permits its distribution without direct financial compensation, it may include specific licensing terms, and may be either open-source or closed-source.
If you're looking for open-source software, use the tag open-source. The developer of open-source software can distribute it gratis or require payment.
FOSS (Free and Open-Source Software) and FLOSS (Free/Libre and Open-Source Software) software are both gratis and open-source.
See also: gratis versus libre in Wikipedia