I have an amount in box2a on 1099-DIV that I don't think should be taxable - is it?

Yes, the amount is taxable and Form 1099-Div, Box 2a reflects total capital gain distributions (long-term). If you didn't get a check for the amount in Box 2a, it means you've directed the payor to reinvestment the money back into your account.

I did not get a check and this gain was reinvested. That being the case, it is still taxable?

Yes, even if you didn't get a physical check for the total capital gains distribution, you still got the money. It just got deposited into your account and is taxable. It's the same with a savings/checking account where you earn interest and the bank just puts it in your savings/checking account.