I stayed in Blanefield for a few weeks two summers ago. From my experience the fishing in the Gareloch wasn't very good.
If you want some better fishing it's worth the trip to Oban or Fort William. The mackerels are starting to run in July. I caught some nice ones near Fort Williams and some cod further down the coast. Loch Long would also be a better choice than Gareloch - at least in my opinion.
Just get some basic spinners. I think I only used silver spoons. There was lots of seaweed at the shoreline. Sometimes a bit difficult to get the spinner back through that last bit.
Or if you if you like fresh water fishing, Loch Lomond could be worth a try for Pike and trout. It could get expensive though since you'd need a license for fresh water fishing.
And the midges were extreme around Loch Lomond in July. They just swarm you as soon as you get out of the car and then there is no escape.
Despite the midges, I love this part of Scotland. Let us know how you get on with your fishing.