This answer assumes that with the term electric gun that you mean a electric gun, which is also called a Taser.
Is it permissible to bring an electric gun in a hoster just in case of self-defense?
With the exception of Austria, such devices are generally forbidden.
In many, the police were allowed to use them, but that is now often restricted.
Therefore for Belgium, you would need a permit issued against the Weapons Act 2006.
The chances for such a permit, to also be carried openly in a holster, for the general purpose of self defense is very unlikely
- such permits are mostly only granted when the applicant is deemed by the authorities to be in great danger
Sources:
United Kingdom
Tasers are considered "prohibited weapons" under the Firearms Act 1968 and possession is an offence.
France
The Taser is classified as a Category B weapon (as is, for example, a 357 Magnum with a barrel: a weapon subject to authorization).
Belgium
The possession and carrying a taser by an individual is banned, as defined in the Weapons Act 2006
Germany
Distance electro-pulse devices (such as Airtaser) are generally prohibited in Germany since April 1, 2008 and are not eligible.
Switzerland
In Switzerland, the transfer, acquisition, transfer and transfer of electroshock devices "that can impair people's resilience or permanently damage their health" is prohibited.
Austria
Under § 1 Z 1 WaffG 1996 is thus confirmed that z. B. o. A. Stun gun "[...] is essentially designed to eliminate or reduce the ability to attack or defend people by direct action [...]" and that it is not a prohibited weapon gem. Paragraph 17 (1) is worded as follows: 'Forbidden is the acquisition, importation, possession, and keeping of arms, the form of which is capable of simulating another object, or which are disguised with objects of daily use;'
Article 5
It is forbidden the marketing, trade, ownership and use, except by state agents specially allowed, and in line with the dispositions of the several regulations of:
[..]
c) electric defenses1, rubber defenses, tonfas or similar
1 And yes, electric defenses include electric guns.