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I played a game as Black in which I fell victim to a gambit trap and soon resigned. Stockfish labled it as the "Tennison Gambit."
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1. Nf3 d5 2. e4 dxe4 3. Ng5 Nf6 4. d3 exd3 5. Bxd3 h6 6. Nxf7 Kxf7 7. Bg6+ Kxg6 8. Qxd8 Nc6 9. Qxc7 Nd5 10. Qg3+ Kh7 11. O-O e5 12. Qd3+
How should I play on after I accept the gambit, and how can I refute the gambit?
There is a saying that "the best way to refute a gambit is to accept it", attributed to Steinitz. While sayings in chess often have exceptions, I think it does apply to this particular case. If you go by either the Stockfish evaluation (-0.8) or the results of games between highly rated players (67% win for black out of 12 games), the gambit is a mistake.
The purpose of a gambit is to trade material for piece activity and the initiative. If you accept the gambit, your plan is usually to brace for the onslaught; if you survive the attack while keeping your material advantage, you can get a winning endgame. That means you have to play solidly and be especially careful about wasting time.
In addition to 5...Nc6 as suggested by Chromatix, there are other moves which prevent this attack. For example, a simple 5...e6 also works. It helps block the diagonal leading to f7 and gives an additional flight square to your king. But more importantly, by not moving the h pawn you still keep control of the g6 square. Other moves that work are 5...Nd7 or 5...Bd7, blocking access to the queen, or 5...Qd7, advancing the queen to a protected square. None of these look particularly attractive to me but they do prevent the tactic and let you keep your material advantage.
With that said, this was a blitz game, and I think gambits work better in blitz because there is less time to think about the proper defense, and going for an endgame with an extra pawn can be tricky because you might run out of time first. So another practical way to "refute" a gambit may well be to decline it, if only because players who play gambits usually want you to accept it, and by not doing what they want they may get frustrated, and it is possible that they haven't prepared for the declined variants as well as for the accepted.
It looks like your position fell apart pretty early on, a symptom of poor opening theory. Your King and Queen were exposed to attack within the first few moves, and White ruthlessly exploited that opportunity.
The first move that Stockfish classifies as a blunder (yes, there's more than one) is 5. … h6?? where you attempt to directly attack the advancing knight. But the knight's threat is not countered by that attack, because it can just continue as planned to threaten your queen, pulling your king out of his home square in an emergency defence.
White then sacrifices his bishop to gain your queen anyway - a net material gain of 3 points if you include his lost knight. While you then move your knight to attack the White queen, it's too late as he can simply move it away to safety.
The suggested move is 5. … Nc6 which pre-emptively protects your queen, turning the above sequence into a losing proposition for White; the game would then develop along a different line.
This is in general the better way to defend against attacks - protect the threatened squares which the attacking pieces want to move to, rather than where they currently stand. To do this effectively you must think at least a couple of moves ahead, which takes practice.
