Under deep vacuum, the pressure in a gauge line will approach absolute zero. Absolute zero is the theoretical point at which all molecular motion ceases and the temperature is the lowest possible. At absolute zero, the pressure would be zero.

In practical terms, the pressure in a gauge line under deep vacuum will depend on the specific conditions of the vacuum system, such as the type of vacuum pump used, the size of the vacuum chamber, and the presence of any leaks. However, it can be expected to be very low, usually in the range of 10^-6 to 10^-9 atmospheres (atm) or lower.