'걸 그랬어' doesn't have another meaning. pretty often sentences can be ended with '걸' without '그랬어', and also the meaning is not changed. this is the most common way to show 'regrets in the past' by putting '걸 (그랬어)' at the end of sentences. so that, you can say, '공부를 더 열심히 할 걸' or '공부를 더 열심히 할 걸 그랬어'. here are similar expressions. '공부를 더 열심히 할 걸 그랬나봐' '공부를 더 열심히 했으면 좋았을걸' '공부를 더 열심히 했어야 했는데' all are the same meaning. for negative, '그런 말은 하지 말걸 그랬어' '그런 말은 하지 말아어야 했는데' '그런 말은 안 할걸' '그런 말은 안 할걸 괸해 그랬어' '그런 말은 안 했어야 했는데' '그런 말을 괸히 했어' in here, '안 하다' is interchangeable with' 하지 말다'.

Thanks to both for embellishing the answer. Definitely gives me a better idea of its everyday use. I closed down the question too early, but luckily the comment option is functional again.

In this case, 것 is a modal noun that's kinda hard to get across. As Donna says, you can leave out '그랬어' to sound more regretful. Another similar structure - 공부 더 열심히 하지 그랬어 You should have studied harder. 그런 말은 하지 말지 그랬어 You shouldn't have said that. In these ones, the subject is always the listener. You could omit 그랬어 here, but then you sound a little bit upset.

as you mentioned its exactly should have + done 1) 공부를 더 열심히 할 걸(=것을)/ 그랬어. studied hard / i should have 2) 너 만나기 전에/밥 먹지 말 걸(=것을)/ 그랬어. eaten before i meet you/ i should not have in speaking we often skip '그랬어' part. and drag '걸' sound longer. i dont add 그랬어 much. ex) 공부 좀 더 할 걸... 돈 좀 더 가져 올 걸... (I should've brought more money) or it can be also 그녀랑 더 많은 시간을 보낼 걸 그랬어 i wish i would spend more time with her