This is a common question – the reason why those extra quotation marks are there is because CSV (comma-separated value) parsers rely on quotation marks to group together items that have commas (or tabs/line breaks) within them, and any actual quotation marks need to be escaped (hence the duplicate quotation mark) and then that needs to get wrapped in quotation marks to indicate that that value needs special handling – and thankfully there have been some good solutions.
In addition to the straightforward solution by @augmented above*, there’s a slightly complicated solution that removes only the extra quotation marks. I’ve adapted @kuovonne’s excellent answer below:
Replace
& {Prix Fab Gastier}
with
& IF(LEFT({Prix Fab Gastier}, 1) = '"',
SUBSTITUTE(
MID({Prix Fab Gastier}, 2, LEN({Prix Fab Gastier}) - 2),
'""',
'"'
),
{Prix Fab Gastier}
)
*In a situation where the formula result is used in a lot of places, this can be preferable since you won’t have to deal with extra quotation marks in those other fields, even though you would lose the original quotation mark.
EDIT: Added missing open curly brace, as noted below