Well you're right, the soil depth is a big issue. Most bulbs want to be buried twice or two and a half times the depth of the bulb, with more soil beneath that. Nothing tall will grow in something so shallow, but you could try low growing plants like campanula muralis, aubretia, cerastium tomentosum, provided there's drainage.

Hardy succulents, such as sedums, houseleeks, Lewisia cotyledon hybrids, and Delospermums are a possibility, also with drainage.
Also smaller annuals, such as pansies, Lobelia erinus, violas, sweet alyssum, dwarf snapdragons, Nemesia, Dicentra, million bells (Calibrachoa), Iceland poppy, dwarf Primula polyantha, and English daisy. Annuals will need close attention to watering in dry periods, and may need supplemental feeding, due to the shallow soil.

Calibrachoa would be great in summer - otherwise, if you're in the UK, pansies are about the only annuals you can use this time of year which will flower on and off through winter..

You have many possibilities. The first question is; what do you like? Then go from there. Depth is an issue, freezing & watering - all limiting factors. It will freeze solid if not protected to the demise of anything growing in it. That said, succulents would be perfect. If you are in a temperate zone, consider Aeoniums (Black Rose) & Euphorbias. They would be right at home in your guttering, 2 of my faves. You can also just bring them indoors when the temps drop. Trailing tuberous begonias & Bacopa plants are on my shopping list for the Spring. Check those out as well.

Small bulbs might be OK but I would forget about anything with height as there isn't enough depth of compost to support them. I'd go with low growing hardy succulents such as sempervivums which are very forgiving. Pity you haven't added your county to your profile - depends to some extent on how severe your winters are because nothing much likes being frozen sllid for long.. You could intersperse them with the bulbs - miniature daffodils, crocus chionodoxa for example.

Thanks guys really appreciate your help, ?

I feel weird, now. I could have sworn that Sunflowerjelly had listed their country as the UK when the question was first asked.

Ha ha, don't feel weird,Tug - your brain probably made that assumption without your realising, based on the language used - 'compost' (presumably potting compost) rather than 'soil' or 'dirt' might be a giveaway

Could be, Bamboo. It's not enough that I forget things--my brain also has to make things up on subconscious assumptions. Grumph!!! :/

Oh I don't know - I think your brain subconsciously deduced the person was in the UK because of the language and the way it was used, thereby preventing you from wasting conscious pre frontal cortex effort in thinking about it , is another way to look at it, or that's what I'd tell myself anyway, ha ha...

Lol, I like it! I assumed UK too but was thinking that county would help as southren and western winters here are often less severe than northern and eastern ones.