Babies often build habits for each nap so for instance when my LO was very young he was in the bait of sleeping only in his cot and I had to make sure I ws home for every nap time which honestly was a bit of a drag especially when A times are short, he wouldn't sleep anywhere else as I had not accustomed him to that habit. When he was a bit older than yours he was in the habit of napping in his own cot for his first nap (2hrs) and in the car for his second and third naps (one on the way somewhere , one on the way home) - this made it much much easier to get out and about, I timed leaving the house to give us a couple of mins to get settled in the car and set off and he would be happily asleep in moments, if I left the house a little late he'd be letting me know he was tired and needed to get to the car fast! Whilst that meant going out 5 or 6 days per week at the same time each day was predictable and consistent, and he slept easily in the car seat for those naps, I really wanted to be able to just stay home one day per week and chill out, maybe even have a nap myself, and I found this incredibly difficult. His habit was to sleep in the car for that second nap and when I didn't want him in the car he found it very hard to sleep even though he self settled and slept wonderfully in his cot for the first nap of the day.
The habits we instil stick - it is up to us parents to decide what we need or want (based around baby's needs of course) and implement it.
If or When we need those habits to change it is up to us to help LO with the change and instil the new habit.
LOs will/can also create habits for each care giver so whilst one might feed to sleep or push baby in the pushchair for a nap another caregiver puts them down for independent sleep - and baby will respond accordingly, they know what to expect. It can bee seen when two parents are sleep training together baby may seem to "regress" when the second parent goes in for their turn because baby hasn't yet realised both parents are going to follow the same method.
Starting with the first nap of the day is great - this is where I'd always suggest if it is possible. Baby has had a restful sleep from the night and there is a level of consistency already.
It is really up to you if you continue now with the gentle sleep training or wait a few weeks. The process and time taken is going to be about the same. Babies can and do change their habits but are likely to protest during the change until they get used to the new habit, which your LO will with your support.
The important aspect really isn't how old your LO is but if you are ready yourself for this. If you are not ready we will still be here when you are