When you type in the opening parentheses after "gsub", I see the entire method signature pop up like this:
However, when I try the tab autocomplete in the way you suggested, this is what I see:
Your screenshot cut off what I think is a crucial part of the picture here: if you look at the bottom of the argument list, you'll see "Sepal.Length" mentioned. I'm not sure how, but I think the implementation of the piping operator might be responsible. It was only introduced in 2014, so it's not implausible that there are some lingering bugs. If you want to get the most accurate representation of the arguments, I would suggest looking at the pop up (what you see in my first screenshot) instead of the autocomplete list.
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