A 'Trophy bride' I a beautiful woman some man marries in order to show her of as his wife, he gain a bit of status that way. She gain financial security, so it is a good deal for both of them. The implicit derision is that there is no love in the marriage.
'gift horse' refers to Greek culture, and the incident where the Greeks lifted their siege of Troy, leaving behind them a big beautiful wooden horse for the Trojans to find. The Trojans took it inside their walled city not realizing it was full of Greek Soldiers who later came out and opened the city gates for the army (that had only pretended to leave).
Neither phrase is particularly connected to the sports domain.
