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Greetings from the Academy of S. Gabriel!
You asked essentially how the name 'Gisla of Roanne' would have been written in the 9th century, and specifically what 'connector' would have been used between the given name and the place-name.
The most appropriate language in which to write your persona's name is Latin. The earliest writings in any French vernacular are the Strassburg Oaths of 842, which are written in a very early northern dialect; there's almost nothing else from the 9th century, and nothing in a southern dialect until c.1000. [3] In 9th century Aquitaine Latin was still very much the language of record.
There are at least a couple of ways the name might have been written in Latin. One, much the most common in the later Middle Ages, is to use the preposition <de> with the place-name in the ablative case (which in this case is identical to the nominative): <Gisla de
Rodumna>. The other is to use an adjective based on the place-name:
<Gisla Rodumnensis>. This type seems to be more appropriate for the
9th century, if we may judge by the names of saints associated with
particular places, e.g., <Sancta Maria Remensis> 'St. Mary of
Reims'. [2] (These are the only bynames of this type that I found
that early.)
I also tried to come up with a plausible spoken (vernacular) form of the full name. Since the spoken language was already an early form of Old French, I'm quite sure that the prepositional form would have been used. I'm not sure, however, just what form(s) the place-name might have taken in the 9th century. On the basis of a 4th century <Roidomna> and an 11th century <Rodenna> [1], I'd hazard a guess at <Rodonna>: <Gisla de Rodonna>. I'm not at all sure of the second vowel in the place-name, but by the 9th century the <d> in it was probably pronounced like the <th> in <this>.
I hope that this letter has been useful; if you have any further questions, please don't hesitate to ask.
For the Academy,
Talan Gwynek
[1] Dauzat, A., & Ch. Rostaing, _Dictionnaire Etymologique des Noms de Lieux de la France_ (Paris: 1963).
[2] Delisle, Leopold, ed., _Rouleaux des Morts du IXe au XVe Siecle_ (New York: Johnson Reprint Corporation, 1968 [1866]).
[3] Lodge, R. Anthony, _French: from Dialect to Standard_ (New York: Routledge, 1993).