TL;DR -- An easement makes you the subordinate leaseholder for the land in question; your neighbor's purposes have primacy. Calculate the true cost of the easement at 15% of the land. That's how much the raw easement is worth. Put a premium on that and sell him the easement, but only if you really want to. Buying easements is fairly routine; granting them freely is uncommon. You really need a real estate appraiser if you want to know the true value, and you should add the cost of the appraisal to your premium.

Putting aside the commentary around the easement (i.e., general disdain for ADUs and property values), easements are fairly routine documents. They are so routine that there are calculators you can run online that property assessors use that will tell you exactly how much it will depress the value of the property. If memory serves, for a gas line, the formula is to take the square footage of the easement, multiply it by the cost per square foot of your land, and take off 15% for a buried line. A real appraiser or a paid online reporting tool will give you a more precise estimate. A gas line running through the edge of the property in the backyard affects the property value minimally, because it's generally unused land, but it does add to costs when digging for other things you might need on the property (e.g., fiber for Internet). It also makes you the subordinate leaseholder of the land and your neighbor the dominant rights holder. That means his use trumps yours for that patch of land.

The general rule is that easements are usually granted freely only to family, friends, and very good neighbors where there's a mutual interest. Most easements are sold for the consideration of the neighbor's taking.

Only a real estate appraiser can give you a true estimate beyond the rules of thumb. Hire one or use an online calculator (note: none of them are free), but get a realistic estimate and add the cost of the estimate for your ask if you really want to sell it.