yes
I'm pretty sure you have to get 10 hearts on him, and the way to get the last heart is to get him the rock lobster, you get that at gull island. But you don't get to go to Gull Island until you finish Daren's rainbow. After yo have his 10 hearts, I'm not sure, but I think you have to upgrade your house to level 3 and make your bed a double person bed. (I'm not quite sure if that is required though) I think you have to propose to him at "On the Hook", but I'm also not sure about that. So if proposing to him there doesn't work, try various places he is usually at like his house or Caramel Falls. You could also try just asking him anywhere.
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If your asking about Rock Lobster you can get them on Gull Island. (Then you give the rock lobster to Toby so you can marry him)
No. He is not a marriage candidate.
Gill will only marry the female main character. He will never marry anyone else. Julius, Chase, Jin, Toby, and Calvin can marry a different girl but Owen, Luke, and Gill do not have anyone else that they will marry.
Yeppers.
That depends. He is a marriage candidate, but there are copies of Tree of Tranquility that have a glitch that makes it impossible to marry Calvin. If you do not have one of the glitched games, then yes you can marry Calvin.