To be a good listener, get talking and get listening
There are many ways to get better at listening.
Start trying to talk Gaelic as much as possible with as many people as you can. The more comfortable you become at talking, the more the structures of Gaelic will make sense to you, helping you contextualise familiar and unfamiliar vocabulary. Engage other Gaelic speakers and learners in conversation, even if for a couple of minutes a day. Every little helps!
Listen to Gaelic more often. Listen to 大象传媒 Radio nan G脿idheal, live, on-demand or as podcasts or to Gaelic programmes on other stations. Learn some Gaelic songs and sing along with them. Over time the sounds, vocabulary and structures of the language will get into your head and help you work out what is involved in a listening passage.
Try as many listening passages as possible. You should try and complete a listening exercise at least once a week. Revise any vocabulary you miss out on afterwards as well. When revising vocabulary, say the word out loud a couple of times, that'll help it stick!
Here are some listening revision guides for you to try right now. Listen to the monologues in the guide and the conversations in the tests, answer the questions, then check how you did against the answers given.