Spring gnomes will also appear on a lot that has a garden. My Life states dynasty has four of them from the recent spring.
Edit: Here's some coding for the seasonal gnomes.
Spring
<kCheckForSpringGnomeTime value="6">
<!--The number of sim hours between tries to create a spring gnome 0.0+-->
</kCheckForSpringGnomeTime>
<kChanceForGnomeOnLotWithGarden value="0.02">
<!--The chance of a spring gnome spawning on a lot with gardens suring spring time. 0.0 - 1.0-->
</kChanceForGnomeOnLotWithGarden>
<kChanceForGnomeFromWoohoo value="0.05">
<!--The chance of a spring gnome spawning on a lot when two sims achieve woohoo. 0.0 - 1.0-->
</kChanceForGnomeFromWoohoo>
<kChanceForGnomeFromNewBaby value="0.05">
<!--The chance of a spring gnome spawning on a lot when two sims make/adopt a baby. 0.0 - 1.0-->
</kChanceForGnomeFromNewBaby>
<kChanceForGnomeFromEgghunt value="0.01">
<!--The chance of a spring gnome being found instead of an egg during egghunt. 0.0 - 1.0-->
</kChanceForGnomeFromEgghunt>
Summer
<kCheckForSummerGnomeTime value="6">
<!--The number of sim hours between tries to create a summer gnome 0.0+-->
</kCheckForSummerGnomeTime>
<kTemperatureRequiredForSummerGnome value="90">
<!--The minimum temperature required to spawn a gnome. 0.0+-->
</kTemperatureRequiredForSummerGnome>
<kChanceOfSpawningOnHotDay value="0.02">
<!--The chance of a summer gnome spawning on a very hot day. 0.0 - 1.0-->
</kChanceOfSpawningOnHotDay>
Fall
<kCheckForFallGnomeTime value="6">
<!--The number of sim hours between tries to create a fall gnome 0.0+-->
</kCheckForFallGnomeTime>
<kChanceForFallGnomeFromScarecrowOnLot value="0.02">
<!--The chance of a fall gnome spawning because of a scarecrow being on the lot. 0.0+-->
</kChanceForFallGnomeFromScarecrowOnLot>
<kChanceForFallGnomeFromDeadSim value="0.05">
<!--The chance of a fall gnome spawning because a selectable sim died. 0.0+-->
</kChanceForFallGnomeFromDeadSim>
Winter
<kCheckForWinterGnomeTime value="6">
<!--The number of sim hours between tries to create a winter gnome due to a 0.0+-->
</kCheckForWinterGnomeTime>
<kChanceOnGnomeFromSnowman value="0.01">
<!--The chance to get a winter gnome if there is a snowman on the lot. 0.0 - 1.0-->
</kChanceOnGnomeFromSnowman>
<kChanceOnGnomeFromHeavySnow value="0.01">
<!--The chance to get a winter gnome if there is heavy snow. 0.0 - 1.0-->
</kChanceOnGnomeFromHeavySnow>
So to put this in English it says basically you can get gnomes this way:
Spring: Easter Egg Hunt, having a garden on the lot, woohooing achieved, making or adopting a baby
Summer: Minimum temperature is 90f and all you need is a very hot day to increase your chance of it spawning.
Autumn/Fall: You need a scarecrow, possible (almost impossible), chance of one spawning from a death of a selected sim, but no mention of Haunted House yet this has been confirmed. Wonder why it didn't show up in all that techeze?
Winter: You can get a snowman from building snowmen, (after 30 snowmen built ZERO gnomes until I made the temperature 32f and then one showed up), chance of one appearing during heavy snow (my snow level was set heavy/deep)
Did I read all that teceze correctly or am I missing something? Are there things like this for other gnomes?