I wouldn't sell the benefit. Now if you were to give it to a family member or close friend, and they gave you a return gift as a "thank you ..."

For me, biggest GM benefit is free access to domestic EC. Routinely save $50+ on transcon flights (I don't even know what the price is because I haven't ever had to pay for domestic EC, but $50 seems like a good minimum per leg). Possibly worth more if you travel with a companion ... did so earlier this year, and got EC on a 764 and back on the A330. This is easily worth several hundred dollars towards GM value.

Next is the 100% Skymiles bonus. While Skymiles aren't the greatest, earning 5K on a one-way transcon instead of 2.5K or 3.1K as FO is a big step up. For a transcon, this is 1900 miles more than Silver, so maybe another $20 per way.

After that is Sky Priority, at least getting Zone 1 and quicker check-in/security lines. I don't have GE yet, but I have gotten PreCheck with GM. Hard to put a monetary value on this but it's worth something.

Next is upgrades, not much, but easy to get if you time your flights well. Last trip I was 3/4 on upgrades, not bad, but two were short flights with not much value in getting to sit in F.

Lastly, SkyTeam Elite+, though I've only used that on 2 trips. Lounge access is nice abroad but can be attained through other means.

These are my values, but those are the biggest benefits to having GM over FO or no status.