I doubt if Bitcoin payment gateways truly have tye lowest transaction fee except when using lightening.
One of the major challenges with using Bitcoin as means of payment is the high transaction fee involved and these transaction fee most times if not all the time is actually paid by the buyer and let's just be realistic that as a buyer after purchasing goods at a said price, I wouldn't think it wise to make payments with my Bitcoin because of the transaction fees but I would rather trade my coin and make payments in my local currency, hence I'm saving cost .
I personally don't think using Bitcoin as a payment option will increase your profitability but rather I think it basically broadens your payment option for people who want to make anonymous transactions.
Another option is to use a payment gateway that uses the Lightning Bitcoin system, it will save more. The fees charged will be much cheaper than using the regular Bitcoin network. even the use of lightning network will be faster and more efficient. It will increase the business profitability of any business that does accept bitcoins as payment. But the problem here is about the regulations that are applied. Businesses using bitcoin will be dealing with regulatory issues.
If you exchange to fiat first for payment then no bitcoin adoption is done, it's the same as you use local currency payments as usual. However, regarding the cost of the Gateway for networks in general, it is also not too expensive, there must also be some price adjustments so that you remain profitable and are not constrained by network costs.