Taxi from Thurso to Gills Bay

It is 16 miles to Gill's Bay from Thurso so in the region of £30 would sound reasonable. It may be more bearing in mind that a return fare is not especially likely.
The big company in the area is Highland Taxis- and you can book with them online.

The bus services run from the centre of Town, not from the railway station. You would need to check if they co-ordinate from your train to your ferry.

They are run by two companies- Aarons of Wick run the 914 and 915 buses, and Stagecoach run the 80 and the 280.

I would highly suggest taking the mid morning train from Inverness. Out of the afternoon train the connection, even by taxi is very tight. It work if the train is on time- but not if it is more than about 5 to 10 minutes late.
If you used the Scrabster ferry and used the bus from Inverness that is a protected connection in the event of late running of the bus. There is no such protection (even out of the Inverness bus) on Pentland ferries,

Thank you for your response on taxi fare and the tip on the train time to Thurso. We will use your suggestions.