After reading these two comments I started second guessing myself. I assumed that a gerund and a present participle, which both end in -ing in English and -ndo in Spanish, were synonymous. Apparently at least some grammarians assert that they are different, as does the article that Ricardo cites.
If my quick skimming of this article is accurate, using the appropriate conjugation of estar with a gerund is permissable, even in the preterite.
Back to whether they are different things or the same, SpanishDict.com says,
"The gerund, also known as the present participle, is used to express several continuous actions in Spanish. You can find the gerundio right under the definition when you conjugate a word using our conjugator."
I remain confused.