Here are some Gameshark Cheat codes for the GB version of Harvest Moon GB, not the GBC version:

 Infinite Money 01FFEFB8 + 01FFF0B8 + 0105F1B8 

 Time Modifier 01??82B8 

 Day Modifier 01??83B8 

 Have Super Sycle in ToolShed 0101A7B8 

 Have Super Hoe in ToolShed 0101A8B8 

 Have Super Hammer in ToolShed 0101A9B8 

 Have Super Ax in ToolShed 0101AAB8 

 Have Cow Medicine in ToolShed 0101ABB8 

 Have Milker in ToolShed 0101ACB8 

 Have Brush in ToolShed 0101ADB8 

 Have Water Can in ToolShed 0101AEB8 

 Have Sprinkler in ToolShed
 (Don't Use with Water Can Code) 0101AFB8 

 Have P. Medicine in ToolShed 0101B1B8 

 Have M. Potion in ToolShed 0101B2B8 

 Have Cow Bell in ToolShed 0101B3B8 

 Have Grass Seeds in ToolShed 0101B4B8 

 Have Turnip Seeds in ToolShed 0101B5B8 

 Have Potato Seeds in ToolShed 0101B6B8 

 Have Tomato Seeds in ToolShed 0101B7B8 

 Have Corn Seeds in ToolShed 0101B8B8 

 Have Eggplant Seeds in ToolShed 0101B9B8 

 Have Peanut Seeds in ToolShed 0101BAB8 

 Have Carrot Seeds in ToolShed 0101BBB8 

 Have Broccli Seeds in ToolShed 0101BCB8 

 Have Pick Ax in ToolShed 0101BDB8 

 Have Umbrella in ToolShed 0101BEB8 

 Have Fishing Rod in ToolShed 0101BFB8 

 Have Cow Feed 0101C0B8 

 Have Chicken Feed 0101C1B8 

 **Have Cat For Pet 0101DAB8 + 0100DBB8 

 Have Dog For Pet 0100DAB8 + 0101DBB8 

 Have No Pet 0100DAB8 + 0100DBB8** 

 Infinite Turnip Seeds 0163D2B8 

 Infinite Potato Seeds 0163D3B8 

 Infinite Tomato Seeds 0163D4B8 

 Infinite Corn Seeds 0163D5B8 

 Infinite Eggplant Seeds 0163D6B8 

 Infinite Peanut Seeds 0163D7B8 

 Infinite Carrot Seeds 0163D8B8 

 Infinite Broccli Seeds 0163D9B8 

 Infinite Grass Seeds 0163DAB8 

 Items Equipped on Position 1 Mod 01??F8B8 

 Items Equipped on Position 2 Mod 01??F9B8 

 Items Equipped on Hands Position 01??FAB8 

 Infinite Crissant 0163C5B8 

 Infinite Rice Ballling 0163C6B8 

 Infinite Meat Dumpling 0163C7B8 

 Infinite Wild Grape Juice 0163CAB8 

 Infinite Green Tea 0163CBB8 

 Infinite Material (Wood) 01FF38B9 + 01FF39B9 

 Infinite Fodder 01FF3AB9 + 01053BB9 

 Events Modifier
(Turn off GS when farmer steps outside) 01??8DB8 

 Weather Modifier 01??A0B8 

 Quantity Digits to Accompany Event Modifier Code 
01 - Get Horse
02 - After Earthquake (This code is useless)
03 - Day Before Picnic
04 - Day of the Picnic
05 - Get Grape Juice
06 - Find A Missing Bird
07 - After Hurricane
08 - Get New Juice
09 - Get Offered a Sprinkler
0A - Get Offered a Lerger House
0B - Find Nina
0C - Girl Stays at Your Ranch 

 Quantity Digits to Accompany Weather Modifer Code 
00 - Clear Weather
01 - Rainy Weather
02 - Snowy Weather
03 - Strong Winds

Credit for these codes go to AMorozin on GameFaqs for finding and testing them. Those with the ** symbols next to them didn't work for him. though, but they may work for others.