From an unused root (meaning to heap up); a stack of sheaves; by analogy, a tomb -- shock (stack) (of corn), tomb.
Englishman's Concordance
1) heap, stack, pile 2) tomb Part of Speech: noun masculine A Related Word by BDB/Strong’s Number: from an unused root (meaning to heap up) Same Word by TWOT Number: 319a, 320a
Brown-Driver-Briggs
I. גָּדִישׁnoun masculineJob 5:26heap, stack (Late Hebrew id., Aramaic id.) — גָּדִישׁExodus 22:5 3t.; — stack of sheaves Exodus 22:5; Judges 15:5 ("" קָמָה in both) Job 5:26.
II. גּדשׁ (= *; exact meaning unknown).
II. גָּדִישׁnoun [masculine]tomb (Arabic id.) Job 21:32; Dr§ 178, ed. 3, p. 229 suggests reading גֶּדֶשׁ.
Exodus 22:6 HEB:קֹצִים֙ וְנֶאֱכַ֣ל גָּדִ֔ישׁ א֥וֹ הַקָּמָ֖ה NAS: to thorn bushes, so that stacked grain or KJV: in thorns, so that the stacks of corn, or the standing corn, INT: to thorn is consumed stacked or the standing
Judges 15:5 HEB:פְּלִשְׁתִּ֑ים וַיַּבְעֵ֛ר מִגָּדִ֥ישׁ וְעַד־ קָמָ֖ה NAS: up both the shocks and the standing grain, KJV: and burnt up both the shocks, and also the standing corn, INT: of the Philistines burning the shocks along grain
Job 5:26 HEB:קָ֑בֶר כַּעֲל֖וֹת גָּדִ֣ישׁ בְּעִתּֽוֹ׃ NAS: Like the stacking of grain in its season. KJV: in a full age, like as a shock of corn cometh in INT: the grave the stacking of grain season
Job 21:32 HEB:יוּבָ֑ל וְֽעַל־ גָּדִ֥ישׁ יִשְׁקֽוֹד׃ NAS: [Men] will keep watch over [his] tomb. KJV: and shall remain in the tomb. INT: is carried over tomb will keep
*; exact meaning unknown). 
id.)