The Microsoft BASIC language is similar to DEC BASIC, but the actual implementation is completely different. For example, DEC BASIC compiles the BASIC source code into bytecode as it's entered for faster execution, while Microsoft BASIC is a pure interpreter that simply interprets the source code tokens at runtime. You have to remember how different the memory-constrained 8-bit microcomputers were from a PDP-11. Even if Gates looked at the DEC BASIC source code, there wasn't much there that he could have used.