All very nice and stylish.. Electric fire place! This will be the easiest way to replace your wood fire place. But the cons in most countries is that electric heating is very expensive. Pros are well.. clean and easy. Press a button and set the heat you want. Effects vary and they are not expensive.

If you want to use gas,

then you have some options. If you have a basement directly under the fire place you can make a safe place to store gas bottles and just put a supply line directly above to the gas fire place. Easy to to replace once every few months and lasts for a bit.

If you have no basement then maybe if space permits a safe place somewhere outside on your wall- hidden away form first sight(but easy access to replace) and run some gas lines to the fire place.

Last option is to keep the gas bottle somewhere in the same room in a safe,ventilated cupboard.. like some people use the a gas stove with a supply bottle right under it. This is the last option (maybe easiest) but safety is a big concern though.

propane(gas) bottles or cylinders.

Propane tank

There is no way to convert a gas fireplace to burn wood, unless you are very lucky and the gas fireplace is an insert in a properly functioning solid fuel fireplace (in which case you basically just remove the insert, and have the fireplace inspected).

In situations like this, you have a couple options.

Install a Solid Fuel Stove

This option will require the least amount of work, though can still be quite expensive and time consuming.

You'll have to follow all local codes that deal with this type of project, but for this example we'll use National Fire Protection Association (NFPA) standards (NFPA 211 Standard for Chimneys, Fireplaces, Vents, and Solid Fuel-Burning Appliances).

Clearances

You'll need 36" clearance from combustible materials on all sides (top, left, right, front, and back).

Floor protection

You'll need a floor protector under the unit, that extends 18" in all directions (except up/down, that would just be silly).

Listed Stoves

The floor protection requirements will be provided by the manufacturer, for listed stoves.

Unlisted Stoves

For unlisted stoves, the floor protection requirements are determined by the length of the legs on the unit.

0" - 2"

Stove must be placed on non-combustible floor.

2" - 6"

4" hollow masonry laid to provide air circulation through the layer, covered with 24-gauge sheet metal.

6" and Greater

Closely spaced masonry not less than 2", covered with 24-gauge sheet metal.

Reducing clearances

There are ways to reduce the clearances, by installing special materials around the stove. Check your local codes for methods to reduce clearances.

Stovepipe

You'll need 18" clearance around the stove pipe, as well as a stovepipe thimble where the stovepipe passes through walls/ceilings.

Chimney

If you don't have a proper masonry chimney to attach the stovepipe to, you'll have to use a factory built metal (A.K.A. prefabricated, Class A, all-fuel) chimney. You likely won't be able to use the same pipe used by the old gas fireplace. You also don't want to connect the stovepipe to the chimney used by your other gas appliances (furnace, water heater, etc.).

Build a masonry fireplace

If you've got the cash and time, you could have a proper masonry chimney/fireplace built (or build it yourself). This option will require modifications to the building structure, lots of masonry skills, a healthy bank account, and a good bit of time.