You've learned the がわ as the reading for this kanji, but when its used on its own as a vocabulary word, it is read as そば. Here's a mnemonic to help you remember:

Having a sober(そば) person by your side is important when you're drunk.

Think of other vocabulary words that use がわ. In these words, 側 is a suffix, so it will have another kanji that comes before it.