Hi Joe,
UPDATE 2024-09-18: According to @KapilAnanth-MSFT (see comment above) There was a miscommunication and Basic SKU isn't GA yet. ETA is tentatively November of this calendar year.
I don't think it has finished rolling out.
Please navigate to your subscription in the portal, next click on Settings -- Preview features blade on left. On right, enter "basic" into search box, select Allow creation of Application Gateway Basic V2 SKU, click Register, and then click OK to confirm.
Wait 20-30 minutes. It takes time for it to register, regardless of what the portal tells you, so be patient.
Freshly navigate to Create application gateway in portal. You should now see Basic sku in the Tier dropdown:
https://portal.azure.com/#create/Microsoft.ApplicationGateway-ARM
Please click Accept Answer and upvote if the above was helpful.
Thanks.
-TP