Recently, when I type up substitution rules for an OpenType feature, I get compilation errors because my apostrophes end up as right single quotes in the input window. I have to copy/paste apostrophes from previous rules to circumvent that! Very annoying. It seems to be a Glyphs thing; it doesn’t happen in other apps. Please remove that automatic replacement.
Two other people reported this recently. I thought I was the only one, but at this point it seems otherwise.
Right click > Substitutions > Smart Quotes.