“Only + full infinitive” is a set construction. It points to a surprising or frustrating subsequent event. Sometimes it can be replaced by
the weaker “but + finite verb”.

• I went into town to buy a new suit, only to find that I’d left my wallet at home.
• I went into town to buy a new suit, but found I’d left my wallet at home.

Seemed like the real thing, only to find mistrust.

• Seemed like the real thing, only to find mistrust, love’s gone behind

Once I had a love and it was a gas…

Someone/Something is a gas: used for saying that someone/something makes you laugh a lot. (ES: Una persona o situación entretenida o divertida)

– the party would be a gas