I think it is true that gamma rays can go right through you without hitting you. Not sure what the absorption rate is, but considering 5cm of concrete only absorbs half of a given gamma radiation source, it's likely pretty low.

This paper seems to imply that the half-value of human flesh is around 10cm under 10MeV, but up to 40cm at 100MeV:

https://www.researchgate.net/publication/345815027_Investigation_of_Gamma-Ray_Attenuation_Coefficients_for_Some_Different_Tissues

So I guess the gamma radiation from a gamma emitter in your stomach would be around 25-75% absorbed, depending on your diameter and the emitter. Not great, but still not 100% absorbed like the other sources, at least.

Other person handled a source on it but one way to think of it is like x-rays. Gamma rays are more much more energetic x-rays basically. X-rays are used in imaging becuase they easily pass through less dense materials like your flesh but not as much in dense materials like say bone. The difference in the distribution of x-rays that passed through your body to get to the other side can be used to make an image showing those differences in density. The higher the energy of the light in this part of the spectrum increases the penetration (not necessarily the case for light in general but at this end of the pool yes), more penetration means less of a percentage of the energy is deposited along the path so more makes it through. Though, even a fraction of high energies deposited can still be bad for you if enough is shining on you. But anyway human tissue is mostly not dense and gamma rays are best attentuated by dense materials like a lead blanket or something so gamma rays coming from the inside can escape you. If you swallow beta or alpha the size and charge of the particle makes it much harder to escape you so virtually all of their energy would be deposited inside you. And neutrons are well moderated by the materials your body is composed of so they will also dump a ton of energy in your body. Alpha and neutron radiation get multipliers for the biological effect of the energy they deposit that reflect them having a much worse impact on you than beta and gamma.

Alpha emitters have medical applications in the body if well-targeted and immediately adjacent or inside of something you want destroyed. For instance boron neutron capture therapy was developed which entails providing B-10 in a compound that is selectively taken up by tumors but ideally NOT by healthy tissue. They then shine a particular energy spectrum of neutrons that will penetrate to the level the cancer is in and the neutrons are captured by the boron isotope (which is very very good at capturing neutrons). The resulting nucleus is not stable and releases an alpha particle, ideally causing massive local damage to the cancerous cell while the short range spares healthy cells.

First generation for that treatment when it was devised many decades back botched it though and wound up killing patients even faster (though it was introduced on people with brain cancers that were basically a death sentence anyway which I guess is where people were willing to try anything). The boron containing compound didn’t target the tumor as selectively as hoped for so a lot wound up in healthy tissues, the neutron energy was too low so the neutrons didn’t penetrate to the necessary depth to destroy the tumors deep in the skull but did cause a lot of damage to healthy scalp tissue, etc.

That said it underwent a lot of refinement later that made it a much more effective treatment.