Probably, though as I left the gas elbow joint in position where it comes through the floor ( for future use ) it was easy as pie to disconnect the fire. The operations are:- turn the gas off at the meter, bleed the residual pressure in the pipe off ( if any ) by turning the fire on briefly, then unscrew the slotted cap on top of the gas elbow fitting. Inside will be a slotted brass grub screw. Screw it down inside the elbow until it bottoms - you have just isolated the gas. Turn the gas on at the meter and check that no gas enters the gas fire ( no hiss, no operation ). This is just a check. Turn the gas off at the meter again ( just for safety ). Next, disconnect the gas pipe from the fire to the gas elbow by using a spanner to unscrew the hex nut clamping the pipe into the gas elbow ( it is just like a water pipe - there is an olive on the pipe to provide a seal ). That's it really, and I've really spun it out. I also then turned the gas on at the meter again and drenched the gas elbow with some soapy water to check for any gas leaks. Replace slotted cap on top of gas elbow. Of course, the above is not advice, it's just what I did when I disconnected my fire. You must do what you think is safe.
Yes you need a ventilation grill; moisture can build up in a chimney, from rain etc. Just as cavity walls are ventilated, you must ventilate an unused chimney. I have seen varous attractive vents available which would be suitable for this. Some are adjustable for those winter nights when you don't want hot air going up the chimney. Plasterboard seems fine for closing up the hole.
Up to you; I will say that my chimneys were unused and unventilated for quite some time, and a trip to the loft when I recommissioned them ( I was checking for smoke leaks ) left me puzzled to see brown stains in the lime mortar where the two brick flues joined together to go up to the chimney stack. This brown, dis- coloured mortar was extremely soft and was crumbling. I worked out that rain had come down the chimney, and it must have soaked that part of the flue on a repeat basis, probably reacting with the soot on the inside of the flues to form an acid which rotted the lime mortar. I spent a day raking out the joints in the attic and repointing to assure myself of a good seal, and to stop the remaining soft lime mortar falling out of the joints. So, for belt and braces, put a cowl top on your chimney pot(s), or a half round pot cap or something, to stop the rain getting in. That is a long-term consideration though.
Andy