Then what word do you suppose we should put in the verse to complete the sentence in English (the italic words are gaps in the Greek)?
It is good neither to eat flesh, nor to drink wine, nor any thing whereby thy brother stumbleth, or is offended, or is made weak."-Romans 14:21 (from https://www.blueletterbible.org/kjv/rom/14/1/s_1060021
)
Besides, Paul already said he was talking about holy days too in vs.5. So we know even without vs. 21 that he is talking about the observance of days too.
"5One person regards one day above another, another regards every day alike."-Romans 14:5
Which is exactly why Paul would NOT address the issue were talking about. It's not a matter of a false gospel vs. a true one. He's talking about matters within salvation that are in dispute among believers. He does not leave matters of salvation itself in dispute.