Someone school me on this. I see a Bible advertised with pages which are 36 GSM Paper. I assume this has to do with thickness or weight. I do write in my Bibles & know if the paper is too thin the print comes through to the back side. I find charts & such but nothing which addresses my concern.

As a comparison, typical printer/photo copier paper is 75 GSM. So 36 GSM paper will be thinner at less than half the weight. You'll have a thin, lightweight Bible but probably have issues with translucence and bleed through of the type from one side to the other of the printed page. Another consideration, if it gets stolen in the wrong part of town then the pages might end up as wrappers for "weed" as the thin paper makes excellent cigarettes, so I'm told.

Thank Paul. That give me something to go on.

...designed, taking those thinner Bible pages into account. Supposedly, they won't smear or "bleed through."

https://jamedders.com/the-best-pens-highlighters-for-your-bible/

https://darrylburling.com/what-is-the-best-way-to-highlight-my-bible/

https://www.amazon.com/bible-highlighters/s?k=bible+highlighters

In the past I've gotten along just fine using colored pencils. Just don't bear down hard on them and lightly trace the area you want highlighted. High quality print shouldn't bleed through even the thin paper that some Bibles use - but in others the print DOES bleed through, "ghosting" and making it hard to read at times, depending on light. It's even worse when the print's small unless you've got nearly perfect vision (compared to mine)