You need to create hidden fields in your gravity form and check “dynamically populate” – then use the GF to SF add-on plugin to map these hidden fields to custom API names you created as new Custom Lead Fields in SF.
Has anyone figured this out? I am having the same issue using the Feed method. I just thought of something that may solve it – if I am only able to feed in text to SF, then I could potentially create a trigger for a new field that would check that original custom lead field as “checked”. Anyone try that yet?