Gillespie, who scored four goals in his last five matches, has undergone an operation on his broken arm and will miss the final two games of the season.
Midfielder David Perkins is on the mend after three weeks out with a hamstring injury but the trip to Somerset may come too soon.
Centre-halves Magnus Okuonghae (ankle) and Pat Baldwin (Achilles) remain long-term absentees while winger Simon Hackney has a hamstring strain.
Boss John Ward is already planning for next term and has told winger Medy Elito, midfielder Sam Corcoran and defender Morten Knudsen they will be released at the end of the season.
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