GPA rounding 2.498 to 2.5. How ethical is this?
There's no way to know how someone will react to a given situation but I would suggest to at least have a reason if you are challenged.

For instance, it might be reasonable to supply the same number of digits that they are asking for. If they ask for a 2.5 minimum GPA, then you report a 2.5, if they ask for a 2.50 then report a 2.50, but if they ask for a 2.500 then report a 2.498. If there is no reference number then I would assume the precision is up to you and report a 2.5.

That being said, I doubt anyone would think anything unethical happened over a .002 rounding that follows all the normal rounding rules.