The Fine Rolls entry mentioned by HOP is viewable on internet archives. It is from "Calendar of the Fine Rolls, Vol.XIV, Henry V, A>D> 1413-1422'", HMSO, 1934, pp. 175-6,
https://archive.org/details/calendaroffiner14greauoft/page/174/mode/2up. It is an order to the escheator of 4 November 1416. It tells the escheator to give seisin of the property of Maurice Russell (deceased wife Isabel) at Durham (Dyrham), Gloucestershire to Maurice's daughters and their husbands: Margaret and her husband Gilbert Denys knight; and Isabel and her husband John Drayton knight.
I have tracked down one of the other references on HOP. It is on the Mapping the Medieval Countryside website, often a good place to look for IPMs. It is C 138/17/61 mm. 1-2 -
https://inquisitionspostmortem.ac.uk/view/inquisition/20-488/ - and relates to the partition of Durham between Margaret & Gilbert Denys, and Isabel & John Drayton.
Other entries relevant to Gilbert Denys on Mapping the Medieval Countryside that I have found are. On a quick glance, none of them cast light on Gilbert's parents, but they confirm his marriage to Margaret Russell and his being father of Maurice.
- E 149/107/12 -
https://inquisitionspostmortem.ac.uk/view/inquisition/20-686A/ (1416 IPM for Maurice Russell, names daughter Margaret as wife of Gilbert Denys)
- C 138/63/27 mm. 1-2 and E 149/126/5 m. 1 -
https://inquisitionspostmortem.ac.uk/view/inquisition/21-933/ (Gilbert's own IPM of 1422, mentions a kinsman Nicholas Denys and names son Maurice, 14 and more, as heir)
- E 149/126/5 m.2 -
https://inquisitionspostmortem.ac.uk/view/inquisition/21-933A/ (order to the escheator to take an oath that Gilbert's widow Margaret would not marry without royal permission, and then deliver her dower: his son Maurice was a minor)
- C 138/27/43 mm. 1-2, E 149/110/11 and C 138/27/43 mm. 3-4 -
https://inquisitionspostmortem.ac.uk/view/inquisition/20-784/ (IPMs of John Drayton, husband of Isabell Russell - refer to the partition of Dyrham between his wife Isabel and her sister Margaret)
- C 139/56/53 mm.1–2 -
https://inquisitionspostmortem.ac.uk/view/inquisition/23-721/ (1431 proof of age for Gilbert's son Maurice)