Vocontia is a local power in the south of Gaul.
Vassal swarm of Gaul
Gaul has huge number of small tribes, sharing the same culture group and religion and this combination allows Vocontia to offer feudatory status to most of them. Feudatory is by far the best vassal type, because it can be integrated, is called in wars and does not cost diplomatic relations slot so their number is unlimited. Subject loyalty is calculated only from difference in power between the single vassal and the overlord, not counting the complete power of all other subjects against the overlord. This is an advantage because all theese small feudatories will usually stay loyal.
At the start Vocontia should make some early alliances to allow easy conquest of the province of Vocontia. Good picks for allies are Rutenia, Allobrogia, Caturigia or Albicia. The player should also move tha capital to Avennio and build city there when possible so its population can grow from the start. The first expansion targets are: Helvia, Arecomicia and Alluvia, the mission "Matter of Transalpine Gaul" should provide the wargoals.
Civilisation, Centralisation, Technology
After first conquest Vocontia should focus on generating research points and raising its civilisation value. To help with technology it is necessary to increase ratio of citizens in the nation, so the player should build many Library buildings as well as an Academy in the capital. One can concentrate the population in the capital by using centralization policy and then promote them to citizens by using social mobility policy. It is important to employ the best characters in their field as researchers and to boost research by all possible bonuses. This way the player should get to solid technology levels, while the individual inventions are quite cheap. It is possible to build new cities, but its not advised to build them in territories, which produce food or mineral resources. (its better to build farming settlements and mines there instead.) However one can try building cities in food-producing territory with hope that it changes its trade good to something valuable. (like papyrus or silk) Civilisation limit can be raised by civic technology, importing goods like Marble, Glass or Gemstones and by using State Religion national idea. It is also necessary to enact laws increasing centralization when they become available.
During this time one can take opportunities to annex other land in the region of Transalpine gaul, but its not advised to expand beyond its borders yet. Before taking the mission to reform into Monarchy or Republic its well advised to have high stability, enough political influence, civilization value of 50 in the capital and 80 ruler popularity. After reform is finished the situation in the game can differ a lot, but with high probability the Romans (or Carthage) will become threat. But Vocontia should at this point be quite powerful too.
Now it is a good time to start building good relations with tribes of Gaul. One should stay our of wars, to not give Romans excuse to attack before Vocontia is ready for them. One should use multiple ways to improve the diplomatic reputation and improve relations maximum. Using Mercantile stance usually gives the best bonus to relations. If Vocontia is strong enough it is quite easy to convince the tribes to become feudatories - first alliance should be offered, then relations improved and/or gifts sent, guaranteeing seems to worsen the chances. It is easily possible to build an army of 40 feudatories this way. Large tribes can be forced to release nations in a peace deal, which does not add up to Vocontia's aggresive expansion.
After this is achieved, it is very easy to expand anywhere and to use Rome as slave farm ready for harvesting. It is possible to form Gaul after integrating feudatories holding critical territorries.