I also have Minolta experience but it applies to the Gossen also.

A couple of years ago I bought a used Minolta that came with 5 degree attachment and also the booster probe. Like David says the probe can be used indoors on groundglass but is near impossible in bright room light. I have never found it to be really useful and I just play with it. I certainly wouldn't pay for another one but I am thinking about making a densitometer with it.

The 5 degree was OK, but after I got a 1 degree meter I will never go back. I now have a Sekonic with 4-1 degree zoom spot and can only remember using the 4 degree setting once.

I also disliked having to carry around and remove and replace attachments to go from spot to incident and back. IMVHO you should get an all in one like the Sekonic, or even better would be to add a dedicated spotmeter to your Gossen.