Gemtext has no commenting feature, however there is a workaround I can think of for "commenting" single lines, by turning the line into the alt text of an empty preformatted text block:
This is a paragraph / other line
becomes
```This is a paragraph / other line
```
I have tested this in lagrange and kristall and it becomes completely invisible.
As the specification doesn't require the alt text to be hidden, there is no guarantee some visitors won't see it.