that 5 gallon per frog rule only applies to certain species. As for grey tree frogs. I would put 2 in that tank at max. 3 is pushing it.
A typical 10 gallon tank is: 20" long x 10" wide x 12" tall. An exto terra mini/tall 12” x 12” x 18” (WxDxH). So yes it is "10" gallons and they are similar. For the comfort of your frogs. I would suggest getting a 20 gallon vert. (18” x 18” x 24” [WxDxH]) Not a mini.

This would also be determined by the age of your frogs. Baby /juvenile frogs can be housed in a mini. Adults I would get the exo-terra small/tall for 3