Zeal Gigas

my opponent has f/d solemn judgment, and evigishki zeal gigas.
i have mst.

Zeal Gigas's text:
Cannot be Normal Summoned or Set. Must first be Ritual Summoned. You can Ritual Summon this card with any "Gishki" Ritual Spell Card. Once per turn: You can pay 1000 Life Points; draw 1 card, and reveal it. If it is a "Gishki" monster, shuffle 1 card on the field to the Deck.

->he activates zeal gigas's effect.
->i chain mst.
<-mst destroy judgment
<-he draw gishki monster
(?)can he then choose the used mst ?(?)

No, it's similar to raiza targetting the bottomless that hit it. You cannot return a "spent" s/t (normal spell that was activated and is to go to the graveyard/normal trap that was activated and is to go to the graveyard etc.) to the hand/deck with cards that simply return to the hand deck except with cards that do specifically that (goblin out of the frying pan, spell reclamation, etc. note however that there are big differences there, spell reclamation will return it after it was sent to the graveyard, goblin negates the activation).
Yes, they can be banished/destroyed instead but not 'spinned' or 'bounced', the reason for that is BKSS, konami does not want you to be reusing your spell cards through such a simple loophole.

OCG ruling wrote:

If "Raiza the Storm Monarch"'s effect targets a face-down Quick-Play Spell Card or Normal Trap Card, and the targeted card is activated in chain to "Raiza the Storm Monarch"'s effect, the targetted Spell/Trap Card is destroyed and not returned to the Deck. If the targeted card is a face-down Continuous Trap Card and it is activated in chain to the effect, then the card is returned to the top of the Deck.[2]

Wouldnt he need to return his Zeal then coz Gigas effect is mandatory

assuming if he can't even choose mst, then yeah, zeal will be returned to deck.
but yeah, there's still a possiblity that zeal can stil choose mst regardless of it not being able to return it to deck.(unless there's a similar card ruling saying otherwise).

Well, Zeal Gigas doesn't target to say he wasted the effect, it is mandatory to return a card so it should select a card that can return. I'll look it up, but he probably returns himself. Hmm, a returning effect that doesnt target and can create a situation where you would not want to return. This can't possibly be a pain to look up...

EDIT: Returning a s/t to the hand without targetting or returning them all is a paaaainfully specific case that I cannot find relevant examples for. Heck, I have a hard time applying this to situations like banishing + IIW or grave + necrovalley. I'll have to get on it when my mind is clearer. I suggest asking somewhere around here http://www.pojo.biz/board/forumdisplay.php?f=15 or managing to bring it up as a general rule and ask in q&a in case someone has a definite answer. That being said I am still going to go with "you cannot choose to waste a mandatory effect", it's understandable with targetting but here it is a different concept.