In Earth politics it may be possible to leave an alliance unilaterally. Most treaties of alliance have clauses describing the conditions for leaving the alliance, usually giving each party the power to leave the alliance unilaterally within a certain period after giving notice of their intention.
But the Galactic Republic in Star Wars is much more similar to an independent state on modern Earth than to an alliance like, for example, Nato.
The Star Wars Galactic Republic seems to resemble a federal state or federation, in which power is shared or divided between the central federal government and the governments of the various parts of the state, but which is legally a single independent and sovereign government in international affairs.
A federation (also known as a federal state) is a political entity characterized by a union of partially self-governing provinces, states, or other regions under a central federal government (federalism). In a federation, the self-governing status of the component states, as well as the division of power between them and the central government, is typically constitutionally entrenched and may not be altered by a unilateral decision of either party, the states or the federal political body. Alternatively, federation is a form of government in which sovereign power is formally divided between a central authority and a number of constituent regions so that each region retains some degree of control over its internal affairs. It is often argued that federal states where the central government has the constitutional authority to suspend a constituent state's government by invoking gross mismanagement or civil unrest, or to adopt national legislation that overrides or infringe on the constituent states' powers by invoking the central government's constitutional authority to ensure "peace and good government" or to implement obligations contracted under an international treaty, are not truly federal states.
At the present time there are 27 federations or federal states listed out of approximately 195 independent states in the world. Most of the largest and/or most populous nations are federations.
Most sovereign states are unitary states with a single government for the whole state.
A unitary state is sometimes one with only a single, centralised, national tier of government. However, unitary states often also include one or more self-governing regions. The difference between a federation and this kind of unitary state is that in a unitary state the autonomous status of self-governing regions exists by the sufferance of the central government, and may be unilaterally revoked.
Another type of state is a confederation.
A confederation, in modern political terms, is usually limited to a permanent union of sovereign states for common action in relation to other states. The closest entity in the world to a confederation at this time is the European Union. While the word "confederation" was officially used when the present Canadian federal system was established in 1868, the term refers only to the process and not the resulting state since Canadian provinces are not sovereign and do not claim to be. In the case of Switzerland, while the country is still known as the Swiss Confederation (Confoederatio Helvetica, Confédération suisse) this is also now a misnomer since the Swiss cantons lost their sovereign status in 1848.
If the Galactic Republic in Star Wars was a confederation of independent and sovereign states for common action with regard to other sovereign states then each of the planetary, star system, or interstellar governments which joined the Galactic Republic would have the right to unilaterally withdraw from the Galactic Republic and there would be an official procedure to do so. Therefore it would be unlikely for separatists to risk civil war by illegally seceding instead of by seceding by the legal method.
The only confederation existing at the present is the European Union, and it is legally possible for a member nation to unilaterally withdraw from the European Union, as the United Kingdom is doing now via "Brexit".
Therefore, I deduce that the government of the Galactic Republic has no legal method for planetary, star system, or interstellar, governments to secede from it. Therefore, the government of the Galactic Republic should resemble either a federation or a unitary state (perhaps one with a lot of autonomy for member governments) instead of a confederation.
Note that the Galactic government is not called the Galactic Union, the Galactic League, the Galactic Confederation, or the Galactic Federation, but the Galactic Republic. Thus there is nothing in the name of the state to indicate whether is is a unitary state, a federation, or confederation.