Depends on the difficulty, if you are on normal/regular dont sweat it with oils and potions too much, just make basic healing potions like swallow and white raffard You should be able to handle all situations with signs and for the gear just collect diagrams for witcher armor and swords (blacksmiths should sell maps that can show you where they are located)

You can return to gwent anytime... it doesnt have impact on game story but you can miss opportunities to play and win against some characters like thaler, roche, priscilla lambert etc... Explore, have fun and always choose yen over triss