Do you need to get specific transformers-converters to run a cordless telephone and halogen lights from Europe?
Yes and no. European appliances run off 220V whereas American appliances run at 110V. However, American homes are wired for both 220 and 110 volts. 220 is just only used for large loads like ovens and dryers. If you know what you are doing you can install 220V outlets and then no, you don't need voltage converters. However, this is a very big job and is not worth the expense if you only have a few European appliances. You can buy 110-220V converters for this purpose.
Quite honestly, there is no specific gas. Granted there are some chemical reactions that would create gases with halogens and H2O, but since you're being very vague on the specific halogen, I can't provide a specific answer. Not all halogens have large reactions with water. In fact, the government forces the American water supply to contain Fluorine (a halogen) to improve the dental health of the country.
All halogen bulbs are incandescent bulbs but not all incandescent bulbs are halogen bulbs! An incandescent bulb has a filament that heats up and gives off light. in the process, some of the filament evaporates. In addition a halogen bulb has halogen gas. When the bulb is turned off, the halogen gas returns atoms from the filament back to the filament. As a result the filament can last longer.