I’m afraid it is not a matter of free vs PRO account types. You can open .ai files but the Illustrator files have to be saved with the option “PDF compatible file” checked.
If this is not the case, you can export an SVG or a PDF from Illustrator and import it back into Gravit Designer, or if you don’t have Illustrator available, you can also use an online tool like https://cloudconvert.com/ai-to-svg.