Gillanders is joined in the matchday 22 by club colleague Ben Youngs, with the scrum-half starting on the replacements bench for the 9.10pm kick off at the Arms Park.
Ben will be hoping to go one stage further than his brother Tom, who helped England win the third-place play-off when the tournament was held in Dubai in 2006.
England are unbeaten in eight games at U-20 level this season, scoring 38 tries in total, after completing a Six Nations Grand Slam and beating Fiji (41-17), Canada (60-18) and Australia so far in the Junior World Championship.
New Zealand take on hosts Wales in the other semi-final at Swansea’s Liberty Stadium (kick off 7pm), with both games being shown live on BBC Wales.
England U-20 (vs South Africa, Wednesday, June 18, Cardiff, kick off 9.10pm)
15 Alex Tait (Newcastle Falcons)
14 Noah Cato (Saracens)
13 Luke Eves (Bristol Rugby)
12 Jordan Turner-Hall (Harlequins)
11 Miles Benjamin (Worcester Warriors)
10 Alex Goode (Saracens)
9 Joe Simpson (London Wasps)
1 Nathan Catt (Bath Rugby)
2 Joe Gray (Northampton Saints)
3 Alex Corbisiero (London Irish)
4 Ben Thomas (Saracens)
5 Gregor Gillanders (Leicester Tigers)
6 Matthew Cox (Worcester Warriors)
7 Calum Clark (Leeds Carnegie)
8 Hugo Ellis (London Wasps, captain)
16 Scott Freer (Leeds Carnegie)
17 Billy Moss (Bath Rugby)
18 Scott Hobson (Cornish Pirates)
19 Jon Fisher (London Irish)
20 Ben Youngs (Leicester Tigers)
21 Rob Miller (Newcastle Falcons)
22 Mark Odejobi (London Wasps)