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Gravit is completely free to use.
It was open source up until a recent release, however there are plans to open source it again in the future.
Pro Multi platform
Works on Windows, Mac and Linux.
Pro Actively developed
Pro Has almost all of the features of more expensive applications
Has comparable features with applications that normally cost a monthly fee.
Pro Symbols and libraries
With Gravit you can re-use the same design for buttons, inputs, etc, in multiple places, with changes synced every where the element is used. It's also possible to set text to a shared style to sync text changes throughout the app.
Pro Clean and helpful UI
The panels in Gravit adjust to contain the information needed based on the type of element selected, preventing the clutter that Photoshop often experiences.
Pro Easy to design multiple pages
The ability to assign layers to specific pages, and view single or multiple pages at once makes it easy to design an entire website without getting lost in thousands of layers. Easy to view multiple pages at one time.
Pro Easy to export all assets
Gravit has a built in exporter to PDF, JPG, PNG and SVG.
Pro Able to open Sketch files
The ability to open Sketch files makes it easier for Linux/Windows users to collaborate with people who use Sketch, which is macOS specific.
Con Changing rapidly
The fast rate of growth/change for Gravit can be a pro in that in means plenty of new features.
However, it also makes the product fairly unpredictable.
For example: In the past year, Gravit was open sourced, then closed sourced again.
Con This does not appear to be a Web-design tool
Gravit appears to be a vector-art tool, and nothing more. Their homepage doesn't even mention Web design as far as I can see.