The Olympus flashes are very popular as they are equivalent to the Panasonic ones. So Oly FL-36 is the same as Panasonic FL360.
Also Metz flashes with Oly mounts.
Panasonic's own flashes are expensive hence the thread. I've never used Nissin Flash but they look interesting. I like having some Flash fill even in some day shoots, if that even means anything. I get a horrible horrible shadow from the 14.140 lens and its worse with the tulip hood.
Slow sync speed of Panasonic bodies will disappoint you. It works fine in low lighting. But the daylight sun affects the overall exposure a lot when sync speed is slow. You'd need to power up a flash more. It might work but it's more tricky and drains batteries quickly.
You could let a subject to face sun. Use spot metering and AEL. Then it might work without a flash.