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Previous entries:

https://opengameart.org/content/super-dead-gunner-dregfly

https://opengameart.org/content/killer-super-dead-gunner-edition

https://opengameart.org/content/super-dead-gunner-fleabot-enemy

https://opengameart.org/content/super-dead-gunner-reaperbot-enemy

https://opengameart.org/content/super-dead-gunner-new-enemy-grenademorta...

https://opengameart.org/content/super-dead-gunner-batch-3

https://opengameart.org/content/super-dead-space-gunner-merc-redux-platf...

https://opengameart.org/content/super-dead-gunner-batch-2

https://opengameart.org/content/super-dead-gunner-platform-shmup-enemies

Tiles and environment stuff that go with them:

https://opengameart.org/content/space-merc-redux-giant-jungle-environment

https://opengameart.org/content/space-merc-redux-lazer-spikes

https://opengameart.org/content/space-merc-redux-techno-dungeon-backgrounds

https://opengameart.org/content/super-dead-space-gunner-merc-redux-space...

https://opengameart.org/content/stormy-clouds-sidescroller-background-fr...

https://opengameart.org/content/space-merc-redux-platform-shmup-tileset

Some other related stuff can be found from my subs page.