Gulf Oil Corp. yesterday announced that four new gas fields it has been developing off the Louisiana coast since 1975 will reach peak production of 240 million cubic feet a day in late 1978.

A Gulf production subsidiary bought the four gas fields at federal auction in 1975.

All gas which is discovered off shore - whether it be in the Gulf of Mexico or the Atlantic or Pacific oceans - must be sold to the interstate market at federally regulated prices.

All gas which discovered after Jan. 1, 1975, costs $1.44 per thousand cubic feet, although some gas flowing from wells discovered years ago costs less than 30 cents. The average price of natural gas flowing in interstate pipelines is about $1.05.

Through its on-shore and off-shore wells, Gulf produces 1.7 billion cubic feet of natural gas a day. But some of this is from wells whose daily production will decline between today and late next year.

As a result, some of the 240 million cubic feet of gas the new fields will yield next year simply will replace gas production lost between now and then.

Gas fields usually are able to maintain peak production for a fraction of the entire time they produce gas because as gas is drawn from a reservoir, pressure is reduced and both the amount of gas forced out and the speed with which it is pushed out the well slackens.

A spokesman for Gulf said he did not know how much gas the company would be producing when the new offshore Lousiana fields reach top production in late 1978.