I'm most interested in the Evanioidea, but you might also bump into me identifying the more distinctive Ichneumonidae, other wasps, bees, and distinctive beetles and dipterans.

Keys to European Gasteruption species

• van Achterberg and Talebi 2014. Includes most European species, though not all of them, e.g. G. erythrostomum and G. subtile. The picture of a male G. minutum is actually of a distinct species, G. boreale, which is keyed in...
• Johansson and van Achterberg 2016. Seperates G. assectator, G. boreale, and G. nigritarse. If you know a specimen is in this group but can't confirm which of the three species it is, I've created a species complex here on iNaturalist which you can identify it to.
• Tan et al 2021 and Tan et al 2016. Focused on China, but include some species that might be found in eastern Europe and Russia. The 2021 paper has pictures of the habitus, antennae and other parts of a male G. minutum.
• van Achterberg 2013. A Dutch key to species that might be found in the Netherlands, sometimes useful for separating the common species. Like van Achterberg and Talebi 2014, this key predates the splitting of G. assectator (s.l.) into G. assectator (s.s.), G. boreale, and G. nigritarse. There are mistakes in couplet 10: use couplet 13 of van Achterberg and Talebi 2014 instead. Includes G. erythrostomum and G. subtile, as does...
• Bogusch 2021. Not freely available online, but the images and species descriptions can be accessed here under 'treatments'.
• Cassar, Mifsud, and van Achterberg 2021. Separates G. jaculator and G. tanyakronum, a species known from the islands of Malta and Italy.

G. zarudnyi is included in some of these keys under the name G. phragmiticola. G. graciloides (= gracilis) is not included in any of these keys. It is listed on the 2019 checklist for Russia, but only for the far east (Primorsky and Kamchatka) and Kazakhstan.

Identifying Gasteruption jaculator and assectator
G. jaculator is not the only species in Europe in which the females have a very long ovipositor with a white tip. G. assectator is also not the only species in which the females have a short ovipositor. Those two species are the best-known and most successful of their kinds, but in almost all of Europe, there are similar species like G. caucasicum, G. tournieri, and G. undulatum which are also fairly common.

Without a good photo of the back of the head from above, G. jaculator can be indistinguishable from G. caucasicum, G. tournieri, and others. Even with good photos, it can be hard to separate from species like G. diversipes and G. opacum.

G. assectator can be very similar to G. undulatum, and even experts confuse it with G. minutum. G. assectator is extremely similar to G. boreale and G. nigritarse, which I wouldn't personally separate without looking at the ovipositor through a microscope.

European Gasteruption checklists
Fauna Europaea does not have up-to-date lists of the Gasteruption known in most territories. Some species are missing, some species are misnamed, and some species are probably wrongly included.

You may be able to find better checklists online, such as this one for Britain and Ireland, either for your territory or for nearby ones. There are usually more Gasteruption in more southern countries, so checklists from places north of your observation are more likely to be missing species found there.

Some keys roughly describe where the species they feature are found. You can also check for newer observations of species in your territory by searching GBIF.