The gerund/present participle is used in different situations. We use it with estar when we are forming a progressive tense: I am doing something, she was doing somehing, they will be doing something, etc. See this lesson for example on how to form the present continuous/progressive tense.
But there are also other ways to use the gerund/present participle, like the one showed in that lesson for "to say by doing something".
I hope it clarified it for you.
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