Granite uses the same Markdown as Obsidian, so your notes move over as they are.
Open the Granite vault you want to import into (or create a new one).
At the bottom of the vault sidebar, open Tools → Import notes (Obsidian, Notion, Markdown).
Choose your Obsidian vault folder, or a zip of it.
Granite reads the files and shows what it will import, and anything it will skip with the reason. Confirm to import.
What carries over: notes with their wikilinks, tags, callouts, and frontmatter, and .canvas files, in the same folders. Obsidian's .obsidian settings folder is skipped.
Import brings in notes and canvases only. Attachments such as images and PDFs are listed as skipped; add them to the vault afterwards, or link them from Google Drive.
Large vault? Granite reads folders in batches and shows its progress ("Reading files… 1,200 of 4,800"), and doesn't read files it's going to skip, so even big vaults go quickly.