Ward handed the Pilgrims a seventh minute lead - but Doncaster Rovers came roaring back with two Joe Ironside goals sandwiching a deflected Tommy Rowe effort.
United reduced the arrears in the 89th minute when Gyasi was fouled in the box, before picking himself up to score the resultant penalty.
United (3-5-2): Cameron Gregory (Jake Lovelace 46); Tom Leak (Trialist B 65), Michael Bostwick (Zak Mills 46), Brad Nicholson (Will Hall 65); Sam Essien (Trialist A 46), Keaton Ward (Keziah Martin 46), Tom Edge (Martin Woods 46), Sam McLintock (Jordan Richards 46), Ethan Sephton (Ryan Hand 65); Kelsey Mooney (Oliver Wilson 65), Jimmy Knowles (Michael Gyasi 65).
Referee: Darren Drysdale.
Attendance: 822 (162 visitors).
PIC: Russell Dossett.