The disagreement on this thread is what is a reasonable accommodation under the ADA. If you read the post by NoVa RT who looked up public accommodations, a reasonable person can understand that providing a ride to the gate is not reasonable accommodation under ADA guidelines.
Regarding the fee - this paragraph seems to cover it:
Make reasonable modifications in policies, practices, and procedures that deny equal access to individuals with disabilities, unless a fundamental alteration would result in the nature of the goods and services provided.
Providing an exception to the 18 dollar entrance fee would be a nice thing for the CG to have done, but a reasonable person can see it from a business owners standpoint that this type of thing can cascade. So yes - a nice thing to do, but not unlawful not to do.
CG's usually have a staff of few people - not a security detail.
Maybe they need to stay at the CG where reportedly the owner took a work crew and demoed the sidewalk and totally rebuilt it in a few hours.
If you don't like the treatment you received at an establishment, your well within your rights to tell others here and on the multiple review sites.