Submitting your income tax declaration is not mandatory if you have paid German capital gains tax without having a Freistellungsauftrag (order to the bank/broker to apply the 801 € exemption to your account) - in general you have to file a tax declaration in Germany only in situations where you may need to pay more taxes.
If you do not have a Freistellungsauftrag, you pay too much capital gains tax. You can claim that back with a tax declaration if you like, but the tax office doesn't force you to...
From the comments it looks like OP may be talking about a situation where no captial gains tax has been paid to Germany since their bank/broker is outside Germany.
This makes the situation much more complex, and may mean that OP has to file German income tax declaration regardless of the 801 € question (would happen also e.g. with rental income in another country that is taxed there, but enters the German calculation of the tax bracket).
Whenever you have income that is subject to German capital gains tax but this tax has not yet been paid, you have to file an income tax declaration (this can happen, also for completely inner-German income).
But to me it is not yet clear whether the capital gains are subject to German capital gains tax. The answer will depend (at least) on:
- to which extent OP is German income tax subject, and
- which tax treaties apply to what extent.
My recommendation is to consult either the tax office or a tax lawyer - I htink this is outside what can be answered here (though it may be good if OP posts a summary of the results later on).
My experience with German tax officers is that you get fair answers to questions that are to the point and relevant for correctly filing your tax declaration.
- Their legal status is that they have to help you to correctly fill in their forms.
- They are not allowed to advise on tax avoidance and will tell to go to a tax lawyer if they think your question points into that direction,
- but if they spot a mistake to your disadvantage they'll tell you to correct that
- usually, they have no interest in receiving more tax declarations (= more work) than necessary.
If that covers your needs, this way is far less expensive than the tax lawyer. If you need advise on how to structure your capital gains to as to reduce the tax or burocratic load, you need a tax lawyer.