If that database contained purely grandmaster games, and we were fast enough to process all of that information, then you could technically replicate the moves of any grandmaster and become equal to their skill level. However, if faced with a board layout not in the database, you would have to use your own intuition. That, unfortunately, must be learnt and cannot be downloaded.
If you downloaded games from chess computers too, you could play at a higher level than any human, however new board states would again throw you off since you would have to process the board yourself and not rely on the database.
So I guess the answer is that if you downloaded a database of all grandmaster and computer games you could theoretically beat any grandmaster as long as you repeat a game in which the side you are playing as won. Any divergence into a new line will make the database obselete.
As for the tablebase, you would have to hold your ground against your opponent until there were 7 pieces left, and then you would be able to spot the best move for the rest of the game. The issue here is that with 7 pieces left, most grandmasters are nearly flawless in their endgames anyway, and unless you enter the endgame with a small advantage you will be pretty much equal, even with machine skill.