Medicare Supplements, also known as Medigap Plans, are standardized plans that offer the same level of benefits when they share the same letter name. The Centers for Medicare & Medicaid Services states, "Cost is usually the only difference between Medigap policies with the same letter sold by different insurance companies." You can find that quote on page 9 of their booklet
Choosing a Medigap Policy: A Guide to Health Insurance for People with Medicare.
Plan F is often referred to as "the Cadillac" plan, it simply is the most comprehensive plan in that it covers all the co-pays, deductibles, and co-insurances that these plans are designed to cover. The table below, seen on page 11 of the same guidebook, indicates the benefit percentage that the Plan F offers for all the different coverage categories.
Plan G is usually a more economical choice. As you can see in the table below, the ONLY difference between Plan G and Plan F is that the Medicare Part B Deductible is not covered by Plan G. As of 2018, this deductible is $183 PER YEAR and pertains to both physician and durable medical equipment costs.
When a carrier offers BOTH plans F and G, you can directly compare the cost of the two plans for the same benefits. You do this by ADDING the $183 deductible to the ANNUAL cost of Plan G. At that point, you have the same benefits. Compare that cost to the ANNUAL cost of Plan F and you will usually see a meaningful savings with Plan G over Plan F.
In other words - the insurance company will charge you MORE than the actual deductible amount for the right to cover it.