Hello there @rsaito,
When you're composing an email through the Gmail app, if you tap the "..." in the top right corner, do you see an option to "Insert from Dropbox"?
If not, can you just check that you have connected your Dropbox and Google accounts from your Connected apps page here (you'll need to have the same email address for both accounts).
Otherwise, if you're having issues with this or getting any errors, please let me know the steps you're taking and send along a screenshot of the error you get.
Let me know if this helps - thanks!