Ingestible and implantable antennas are widely used in current in-body bioelectronic devices. This paper aims to examine the behavior of magnitude and phase of the reflection coefficient of ingestible antennas in the gastrointestinal tract for the purpose of sensing and distinguishing the gastrointestinal tissues (stomach, small intestine, and large intestine). In this context, the paper presents the changes in these parameters for three common antenna types (dipole, loop, and patch) in the interval of electromagnetic properties (relative permittivity and electrical conductivity) of the gastrointestinal tissues. The antennas operate in the 433 MHz Industrial, Scientific, and Medical Band and conform to the inner surface of polylactic-acid capsules with shell thicknesses of 0.2, 0.5, and 1 mm. They are optimized in a homogeneous spherical phantom having time-averaged electromagnetic properties of the gastrointestinal tissues. The changes in the magnitude and phase are presented in the intervals of 57–72 for the relative permittivity and 0.5-2.1 S/m for the conductivity, covering the values of the gastrointestinal tissues. The results show that the phase varies in wider intervals than the magnitude, mostly due to the changes in the conductivity, indicating that it is better to track the phase for distinguishing the gastrointestinal tissues.