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.

You probably want to read about significant figures. The premise of significant figures is that only so many additional digits mean anything. Depending on how many digits others want to see your GPA at, yes it is ethical to round a 2.498 to a 2.5. It's well known that GPA isn't a precise predictor of any kind of future achievement, otherwise your interviewer might simply be taking the person with the highest GPA and not so much as interview.

Mentioning two digits of your GPA (2.5) is probably enough information for your interviewer. If they ask, your interviewer would probably be thinking much more about the first digits than about whether you are bumping up your GPA to meet some sort of line.

I doubt an employer would perceive you rounding to 2.5 as unethical. However, you need to be aware of how they are using that information.

If you are interviewing for some government or academic appointment, 2.49999 is not "at least 2.5" and could disqualify you from a bureaucratic technical standpoint.

If that's the case, you could end up getting fired. This would be considered misrepresenting yourself and fraudulent.

My best advice is just don't volunteer your GPA. Unless there's some requirement, don't put it on your resume. If asked verbally, say "about 2.5". If filling out some form, put down the exact answer.