If I bought a mutual fund shares through an online brokerage account (Ex tradeking..etc) and the fund gained money, will I get dividends?

You will always get dividends, no matter where the fund is.

In a brokerage account you choose how your dividends are distributed, either by reinvesting in the fund or cash in your account, if you choose to reinvest then you would be paying transaction fees unless they offer a special program. Check your brokerage conditions.

Either way, you will pay tax on your dividends at year end.

In a 401k, your dividends are always reinvested, there should be no fees and no tax until you liquidate your 401k.

Hint: if you are buying a Vanguard fund, sign up in vanguard and buy the fund directly from them, no commission on purchase or dividends.

You will only get a dividend if the stocks in the mutual fund get a dividend, or if the mutual fund managers decide to give a dividend. Read the mutual fund's prospectus.

There may or may not be a commission as it depends on the mutual fund. Again, read the prospectus, or the online brokerage fee schedule.