A GbR is a civil law partnership and principally "tax transparent" for German tax purposes. The partners of the GbR are regularly obliged to annually file a German partnership tax return (einheitliche und gesonderte Feststellungserklärung) within the time period prescribed by law. In the partnership tax return the relevant income of all (German taxable) partners of the GbR (and each share of such income) has to be calculated by applying the respective German (corporate) income tax rules. The respective share of income is subject to German (corporate) income tax on the level of the respective German taxable partner of the GbR.
If the GbR does not qualify as a commercial or commercially imprinted partnership certain income (e.g., certain capital gains) do not have to be included in the partnership tax return but directly in the (corporate) income tax returns of the respective German taxable partners of the GbR.
If the GbR qualifies as a commercial or commercially imprinted partnership (Mitunternehmerschaft) German trade tax accrues additionally on the level of the GbR and the GbR has to file respective German trade tax returns. The share of German trade tax allocable to the German taxable partners may (partly) be credited against the German (corporate) income tax of the respective German taxable partner.
A GmbH is a corporate-type entity and opaque for German tax purposes. The GmbH has, inter alia, to file annually German corporate income tax and trade tax returns within the time period prescribed by law. Income generated by the GmbH is subject to German corporate income tax and German trade tax on the level of the GmbH.